[Documentation] Exception thrown with code copied from documentation

**Note: for support questions, please use stackoverflow**. This repository's issues are reserved for feature requests and bug reports.

* **What is the current behavior?**
The code listed in ["ObjectType -> Quick Example"](https://docs.graphene-python.org/en/latest/types/objecttypes/#quick-example) throws exceptions.

My code:
```python
from graphene import ObjectType, String, Field, Schema

class Person(ObjectType):
first_name = String()
last_name = String()
full_name = String()

def resolve_full_name(parent, info):
return f"{parent.first_name} {parent.last_name}"

def person_resolver(root, info):
return {
"first_name": "John",
"last_name": "Doe",
}

class Query(ObjectType):
Person = Field(Person, resolver=person_resolver)

schema = Schema(query=Query, auto_camelcase=False)

data = schema.execute(
"""
query{
Person{
first_name
last_name
full_name
}
}
"""
)

print(data)
```

Exception thrown:
```
An error occurred while resolving field Person.full_name
Traceback (most recent call last):
File "C:\Users\keith.vandermeulen\Documents\play\venv\lib\site-packages\graphql\execution\executor.py", line 452, in resolve_or_error
return executor.execute(resolve_fn, source, info, **args)
File "C:\Users\keith.vandermeulen\Documents\play\venv\lib\site-packages\graphql\execution\executors\sync.py", line 16, in execute
return fn(*args, **kwargs)
File "test.py", line 7, in resolve_full_name
return f"{parent.first_name} {parent.last_name}"
AttributeError: 'dict' object has no attribute 'first_name'
Traceback (most recent call last):
File "C:\Users\keith.vandermeulen\Documents\play\venv\lib\site-packages\graphql\execution\executor.py", line 452, in resolve_or_error
return executor.execute(resolve_fn, source, info, **args)
File "C:\Users\keith.vandermeulen\Documents\play\venv\lib\site-packages\graphql\execution\executors\sync.py", line 16, in execute
return fn(*args, **kwargs)
File "test.py", line 7, in resolve_full_name
return f"{parent.first_name} {parent.last_name}"
graphql.error.located_error.GraphQLLocatedError: 'dict' object has no attribute 'first_name'

OrderedDict([('errors', [{'message': "'dict' object has no attribute 'first_name'", 'locations': [{'line': 6, 'column': 5}], 'path': ['Person', 'full_name']}]), ('data', OrderedDict([('Person', OrderedDict([('first_name', 'John'), ('last_name', 'Doe'), ('full_name', None)]))]))])
```

* **If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem** via
a github repo, https://repl.it or similar.

* **What is the expected behavior?**
The documentation should contain working code.

* **What is the motivation / use case for changing the behavior?**
The documentation first example doesn't work. This seems like a bug...

* **Please tell us about your environment:**

- Version: 2.1.8
- Platform: Windows 10

Here is my `pip list` command from a fresh virtualenv:
```
Package Version
------------- -------
aniso8601 7.0.0
graphene 2.1.8
graphql-core 2.3.2
graphql-relay 2.0.1
pip 20.0.2
promise 2.3
Rx 1.6.1
setuptools 46.0.0
six 1.15.0
wheel 0.34.2
```

* **Other information** (e.g. detailed explanation, stacktraces, related issues, suggestions how to fix, links for us to have context, eg. stackoverflow)
Please note that I created a brand new virtualenv with a fresh pip install today to try this out. It seems as though `parent` is actually a dictionary and the resolver works if changed to the following:

```python
def resolve_full_name(parent, info):
return f"{parent['first_name']} {parent['last_name']}"
```