gstlal_cherenkov_zl_rank_pdfs: change the comman line options

- rename --candidate-cache to --candidates-cache

gstlal-burst/bin/gstlal_cherenkov_zl_rank_pdfs

@@ -54,17 +54,18 @@ __author__ = "Soichiro Kuwara <soichiro.kuwahara@ligo.org>"
 
 def parse_command_line():
 	parser = OptionParser(
-		description = "Rankngstat calculation program for Cherenkov burst search."
+		description = "Rankngstat calculation program for Cherenkov burst search.",
+		usage = "%prog [options] [candidatesxml ...]"
 	)
-	parser.add_option("--candidate-cache", metavar = "filename", help = "Also load the candidates from files listed in this LAL cache.  See lalapps_path2cache for information on how to produce a LAL cache file.")
+	parser.add_option("--candidates-cache", metavar = "filename", help = "Also load the candidates from files listed in this LAL cache.  See lalapps_path2cache for information on how to produce a LAL cache file.")
 	parser.add_option("-v", "--verbose", action = "store_true", help = "Be verbose.")
 	parser.add_option("--ranking-stat-pdf", metavar = "filename", help = "Write zero-lag ranking statistic PDF to this file.  Must contain exactly one Cherenkov burst ranking statistic PDF object.")
 	options, urls = parser.parse_args()
 
 	paramdict = options.__dict__.copy()
 
-	if options.candidate_cache is not None:
-		urls += [CacheEntry(line).url for line in open(options.candidate_cache)]
+	if options.candidates_cache is not None:
+		urls += [CacheEntry(line).url for line in open(options.candidates_cache)]
 	if not urls:
 		raise ValueError("must provide some candidate files")