steampipe_fixups: Fix usage text to reference the "process" verb.

[wuisabel-gif]

The usage() output documents the first verb as prepare, but the argument
dispatch in __main__ only accepts process (matching the do_process
function). Running the documented command therefore falls through to the usage
branch and exits non-zero instead of processing the directory:

$ steampipe_fixups.py prepare somedir
Usage:
...
$ echo $?
1

The working verb is process; this aligns the help text with it. No behavior
change, only the printed usage string is corrected.