Fix a dead link in doc.

The original link is not available any more.

Change-Id: I0d47f5e05d80973bc29deb377ccec814f555e3ef

docs/source/contributing.rst

diff --git a/docs/source/contributing.rst b/docs/source/contributing.rst
index cedac9b..3ef8629 100644
--- a/docs/source/contributing.rst
+++ b/docs/source/contributing.rst
@@ -16,7 +16,7 @@
 to make a suggestion or three.
 
 We follow Google's `C++ Style Guide
-<http://google-styleguide.googlecode.com/svn/trunk/cppguide.xml>`_ and
+<https://google.github.io/styleguide/cppguide.html>`_ and
 use `git <http://git-scm.com/>`_ for version control. We use the
 `Gerrit <https://ceres-solver-review.googlesource.com/>`_ to collaborate and
 review changes to Ceres. Gerrit enables pre-commit reviews so that