[clang-format][docs] Fix invalid CSS syntax in versionbadge

CSS uses colons, not the equals sign. The final semicolon is optional,
but preferred to be included. Really, the font property doesn't really
need to be there, but I suppose it was put there for a reason.

It's surprising how lenient browsers are when parsing

Reviewed By: HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D138441