[docs] Use monospace for PCH option flags

Summary:
Use monospace for option flags in the PCH section, instead of the
italics that were being used previously.

I believe these used to be links, for which single backticks would
have been appropriate, but since they were un-link-ified in
https://reviews.llvm.org/rL275560, I believe monospace is now more
appropriate, and so two backticks are needed.

Test Plan:
Build the `docs-clang-html` target and confirm the options are rendered
using monospace font.

Reviewers: sepavloff, aaron.ballman

Reviewed By: aaron.ballman

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D42005

llvm-svn: 322447