doc: escape RE with backticks (`)

[lbellomo]

On chmod and on mkdir there is a RE that is valid markdown link:

This line:
Each MODE is of the form '[ugoa]*(-+=)+|[-+=]?[0-7]+'.
Into this:
Each MODE is of the form [ugoa]*([-+=]([rwxXst]*|[ugo]))+|[-+=]?[0-7]+.

I try to escape that RE using monospace/backticks.

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/stdbuf (fails in this run but passes in the 'main' branch)

[cakebaker]

Good catch, thanks!