Minor pg_amcheck fixes spotted while reading code

When perusing the pg_amcheck code the other day for the recently pushed fixes I
found two small things that seem worth fixing: The help function progname
parameter shadows the global variable of the same name, which no doubt will
make static analyzers complain so we might as well fix that; and there is a
tiny typo in the test stanza.

Both fixed in the attached.

--
Daniel Gustafsson        https://vmware.com/