Remove mention of password_encryption = plain in postgresql.conf.sample.

Evidently missed in commit eb61136d.

Spotted by Oleg Bartunov.

Discussion: https://postgr.es/m/CAF4Au4wz_iK5r4fnTnnd8XqioAZQs-P7-VsEAfivW34zMVpAmw@mail.gmail.com

src/backend/utils/misc/postgresql.conf.sample

@@ -85,7 +85,7 @@
 #ssl_key_file = 'server.key'
 #ssl_ca_file = ''
 #ssl_crl_file = ''
-#password_encryption = md5		# md5, scram-sha-256, or plain
+#password_encryption = md5		# md5 or scram-sha-256
 #db_user_namespace = off
 #row_security = on