[clang-format] Fix style of css file paths

CSS files included in HTML should have a path in posix style, it should
not be different for Windows.

Differential Revision: https://reviews.llvm.org/D65309

git-svn-id: https://llvm.org/svn/llvm-project/clang-tools-extra/trunk@367137 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed