[gdb/cli] Fix typo in cli-dump.c

Fix the folowing typo:
...
$ codespell --config gdb/contrib/setup.cfg gdb/cli
gdb/cli/cli-dump.c:400: useable ==> usable
...
and add gdb/cli to the pre-commit codespell configuration.

Approved-By: Tom Tromey <tom@tromey.com>