[clang-doc] Fix failing tests on Windows

Tests on Windows were failing due to path separator differences.
Links in HTML should use posix-style paths.

Differential Revision: https://reviews.llvm.org/D65419

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@367264 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed