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 <[e-mail]>
Reviewed-by: Hendrik Brueckner <[e-mail]>
Acked-by: Masami Hiramatsu <[e-mail]>
Cc: Martin Schwidefsky <[e-mail]>
LPU-Reference: [e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/builtin-buildid-cache.c

···
 		   "file", "kcore file to add"),
 	OPT_STRING('r', "remove", &remove_name_list_str, "file list",
 		    "file(s) to remove"),
-	OPT_STRING('p', "purge", &purge_name_list_str, "path list",
-		    "path(s) to remove (remove old caches too)"),
+	OPT_STRING('p', "purge", &purge_name_list_str, "file list",
+		    "file(s) to remove (remove old caches too)"),
 	OPT_STRING('M', "missing", &missing_filename, "file",
 		   "to find missing build ids in the cache"),
 	OPT_BOOLEAN('f', "force", &force, "don't complain, do it"),