DOC Fix inconsistencies
Fix a few inconsistencies in the docstrings.

Not sure how pandas community feels about these kinds of fixes (and this is my first PR to pandas). I just notice things that are different from how they occur most of the time and decided to fix them up. 

### Examples
#### Add 's' to headers
```
Example
```
to
```
Examples
```

#### Correct number of hyphens under headers
```
Yields
-------
```
to
```
Yields
------
```

#### Rewrite testcase in standard format
```
In [1]: c = pd.Categorical(list('aabca'))
```
to
```
>>> c = pd.Categorical(list('aabca'))
```


