parameter-overrides on command line block samconfig.toml values

### Description:
I have most of my `parameter_overrides` defined in my `samconfig.toml` file for my `local` commands. However, I want to pass a single value via the command line `--parameter-overrides` value. The documentation states:

>For the parameter_overrides entry, both the parameter values that you provide on the command line and entries in the configuration file take precedence over corresponding objects declared in the Parameters section of the template file.

This would indicate to me that you can provide both and they'll be merged but this doesn't appear to be happening.

### Steps to reproduce:

**samconfig.toml**
```toml
version = 0.1
[default.local_invoke.parameters]
parameter_overrides = "myFuncBucket=\"dev-my-func\""
```
Make the following call:
```
sam local invoke MyFunc --parameter-overrides 'ParameterKey=myFuncQueue,ParameterValue=QueueValue'
```

### Observed result:

The value of `myFuncQueue` is passed to the function but the value of `myFuncBucket` is not.

### Expected result:

Both `myFuncQueue` and `myFuncBucket` should be available to the function.

or...

The documentation should be updated to reflect that passing a command line `parameter-overrides` will prevent the use of the `parameter_overrides`.

I can make that documentation update if that is the case. However, I'm hoping that this is a bug.

### Additional environment details (Ex: Windows, Mac, Amazon Linux etc)

```
$ cat /etc/issue
Ubuntu 20.04.1 LTS \n \l
$ sam --version
SAM CLI, version 1.7.0
$ go version
go version go1.15.4 linux/amd64
```