vacuumdb: Reword --help message for clarity

The --help output stated that schemas were specified using PATTERN
when they in fact aren't pattern matched but are required to be
exact matches. This changes to SCHEMA to make that clear.

Backpatch through v16 where this was introduced.

Author: Kuwamura Masaki <kuwamura@db.is.i.nagoya-u.ac.jp>
Discussion: https://postgr.es/m/CAMyC8qp9mXPQd5D6s6CJxvmignsbTqGZwDDB6VYJOn1A8WG38w@mail.gmail.com
Backpatch-through: 16

src/bin/scripts/vacuumdb.c

@@ -1171,8 +1171,8 @@ help(const char *progname)
 	printf(_("      --no-process-main           skip the main relation\n"));
 	printf(_("      --no-process-toast          skip the TOAST table associated with the table to vacuum\n"));
 	printf(_("      --no-truncate               don't truncate empty pages at the end of the table\n"));
-	printf(_("  -n, --schema=PATTERN            vacuum tables in the specified schema(s) only\n"));
-	printf(_("  -N, --exclude-schema=PATTERN    do not vacuum tables in the specified schema(s)\n"));
+	printf(_("  -n, --schema=SCHEMA             vacuum tables in the specified schema(s) only\n"));
+	printf(_("  -N, --exclude-schema=SCHEMA     do not vacuum tables in the specified schema(s)\n"));
 	printf(_("  -P, --parallel=PARALLEL_WORKERS use this many background workers for vacuum, if available\n"));
 	printf(_("  -q, --quiet                     don't write any messages\n"));
 	printf(_("      --skip-locked               skip relations that cannot be immediately locked\n"));