pgsql: Fix incorrect option name in usage screen

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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/0fba25eb720a6a20b8f3c9597dd0417a0d20fc94

Modified Files
--------------
src/backend/utils/activity/generate-wait_event_types.pl | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)