Fix incorrect option name in usage screen

The usage screen incorrectly refered to the --docs option as --sgml.
Backpatch down to v17 where this script was introduced.

Author: Daniel Gustafsson <daniel@yesql.se>
Discussion: https://postgr.es/m/20250729.135638.1148639539103758555.horikyota.ntt@gmail.com
Backpatch-through: 17

src/backend/utils/activity/generate-wait_event_types.pl

@@ -334,12 +334,12 @@ close $wait_event_names;
 sub usage
 {
 	die <<EOM;
-Usage: perl  [--output <path>] [--code ] [ --sgml ] input_file
+Usage: perl  [--output <path>] [--code ] [ --docs ] input_file
 
 Options:
     --outdir         Output directory (default '.')
     --code           Generate C and header files.
-    --sgml           Generate wait_event_types.sgml.
+    --docs           Generate wait_event_types.sgml.
 
 generate-wait_event_types.pl generates the SGML documentation and code
 related to wait events.  This should use wait_event_names.txt in input, or