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.