update_manpages.py: treat regex as raw string

Treat the values in the regex map as raw strings to fix
Invalid escape sequence 'g' (W605).

Change-Id: I53bf5d6bd1e1d6a1d1293e4f55640b6513bf3075
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/354698


Tested-by: Daniel Kutik <daniel.kutik@lavawerk.com>
Reviewed-by: Mike Frysinger <vapier@google.com>