Fix missing text on Index documentation page.
Thanks! However, I think, looking at the fix in sphinx 1.2.2 (https://bitbucket.org/birkenfeld/sphinx/pull-request/218/closes-1374-autosummary-generates-overly/diff), it should be enough to put a full stop at the end of the sentence to have the same effect and not having to go over the 80 char line width.
 Nice, I'll do that now
 @graingert thanks!
