perf buildid-cache: Update help text for purge command

Clarify the perf buildid-cache help text for the purge operation.  The
purge subcommand takes a list of files (binaries) as option parameter.
Make the wording the same as for the add and remove operation.

Signed-off-by: Thomas-Mich Richter <tmricht@linux.vnet.ibm.com>
Reviewed-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
LPU-Reference: 20171107144853.12925-1-tmricht@linux.vnet.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>