Tweak contrib --help output to match common style

Placeholders such as OPTION are typically kept in singular.

contrib/oid2name/oid2name.c

@@ -177,7 +177,7 @@ help(const char *progname)
 {
 	printf("%s helps examining the file structure used by PostgreSQL.\n\n"
 		   "Usage:\n"
-		   "  %s [OPTIONS]...\n"
+		   "  %s [OPTION]...\n"
 		   "\nOptions:\n"
 		   "  -d DBNAME    database to connect to\n"
 		   "  -f FILENODE  show info for table with given file node\n"

contrib/pg_upgrade/option.c

@@ -218,7 +218,7 @@ usage(void)
 {
 	printf(_("pg_upgrade upgrades a PostgreSQL cluster to a different major version.\n\
 \nUsage:\n\
-  pg_upgrade [OPTIONS]...\n\
+  pg_upgrade [OPTION]...\n\
 \n\
 Options:\n\
   -b, --old-bindir=OLDBINDIR    old cluster executable directory\n\

contrib/pgbench/pgbench.c

@@ -338,7 +338,7 @@ usage(const char *progname)
 {
 	printf("%s is a benchmarking tool for PostgreSQL.\n\n"
 		   "Usage:\n"
-		   "  %s [OPTIONS]... [DBNAME]\n"
+		   "  %s [OPTION]... [DBNAME]\n"
 		   "\nInitialization options:\n"
 		   "  -i           invokes initialization mode\n"
 		   "  -F NUM       fill factor\n"