Fix highlighting for raw byte strings

[nico-abram]

highlight.js seems to not render rust raw byte string literals correctly. Example here: https://doc.rust-lang.org/reference/tokens.html?highlight=raw%20string#raw-byte-string-literals (Look for br" )

This pull request should fix the problem in highlight.js, but I'm opening this since mdbook will need to update once a version with the fix is released

The last commit updating highlight.js (9 Jul 2020) has the build options that were used to build it in the description