Update Graph Syntax

After installing age extension, I followed the documentation to create the graph.

website link: [Create a Graph](https://age.apache.org/age-manual/master/intro/graphs.html)

`create_graph(graph_name);` <-- this is the command, to run for creating a new graph, as per the documentation.

If I run command as:
```
postgres=# create_graph(graph_name);
```
### Output
```bash
ERROR: syntax error at or near "create_graph"
LINE 1: create_graph(graph_name);
```

or

```
postgres=# create_graph("graph_name");
```
### Output
```bash
ERROR: syntax error at or near "create_graph"
LINE 1: create_graph("graph_name");
```

or

```
postgres=# create_graph('graph_name');
```

### Output
```bash
ERROR: syntax error at or near "create_graph"
LINE 1: create_graph('graph_name');
```

The result is same.

---

## What worked for me:

I will have to use the command as:
```
postgres=# SELECT create_graph('graph_name');
```

### Output
```bash
NOTICE: graph "graph_name" has been created
create_graph
--------------

(1 row)
```

---

## Question:

Just want to ask if docs can be updated with working command.