DOC: small error in cookbook (doc build warning)
All doc build warnings should now be gone again :-)

(at least, except of one, which I can't get rid off: `pandas\doc\source\_templates\autosummary\class.rst:: WARNING: document isn't included in any toctree`, but normally Sphinx should ignore the files in _template)
 yeh...that last one is odd.....thanks!
 Last one is necessary for redirects from nested (pandas.core.Series) to top
level (pandas.Series)
 @jtratner What do you mean exactly? I think I don't understand it.
 For a while, all the pandas docs were nested (I think) so the autosummary
for Series was under core rather than at the toplevel. The point of that
file is to make sure that previous links to the auto-generated docs for
pandas.core.Series still work, but they aren't added to the public TOC
