Fix broken `Show Source` links on documentation pages (GH-3113) (GH-3125)

The `Show Source` was broken because of a change made in sphinx 1.5.1
In Sphinx 1.4.9, the sourcename was "index.txt".
In Sphinx 1.5.1+, it is now "index.rst.txt".
(cherry picked from commit b9ff498793611d1c6a9b99df464812931a1e2d69)
1 file changed