Contribution Guide - Building the Documentation single function error
#### Code Sample, a copy-pastable example if possible

```
python make.py --single DataFrame.join
Traceback (most recent call last):
  File "make.py", line 339, in <module>
    sys.exit(main())
  File "make.py", line 334, in main
    args.verbosity, args.warnings_are_errors)
  File "make.py", line 46, in __init__
    single_doc = self._process_single_doc(single_doc)
  File "make.py", line 88, in _process_single_doc
    'pandas.DataFrame.head)').format(single_doc))
ValueError: --single=DataFrame.join not understood. Value should be a valid path to a .rst or .ipynb file, or a valid pandas object (e.g. categorical.rst or pandas.DataFrame.head)

```
#### Problem description

Using the code snippet described in the pandas contribution guide (https://pandas-docs.github.io/pandas-docs-travis/development/contributing.html#id48), we get a ValueError. This is fixed by instead using the following command:

```
python make.py --single pandas.DataFrame.join
```

I will create a PR updating the documentation.