[clang-format][docs] Fix the Google C++ and Chromium style guide URLs

Summary: The Google C++ and Chromium style guides are broken in the clang-format docs. This patch updates them.

Reviewers: djasper, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: cfe-commits

Tags: #clang

Patch by: m4tx

Differential Revision: https://reviews.llvm.org/D61256

llvm-svn: 373844