zpool: trim -p is not a valid option

This commit removes the documented but not handled "-p" option from
zpool(8) help message.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Chris Dunlop <chris@onthe.net.au>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes openzfs#8781

cmd/zpool/zpool_main.c

@@ -30,6 +30,7 @@
  * Copyright (c) 2017 Datto Inc.
  * Copyright (c) 2017 Open-E, Inc. All Rights Reserved.
  * Copyright (c) 2017, Intel Corporation.
+ * Copyright (c) 2019, loli10K <ezomori.nozomu@gmail.com>
  */
 
 #include <assert.h>
@@ -384,7 +385,7 @@ get_usage(zpool_help_t idx)
 	case HELP_RESILVER:
 		return (gettext("\tresilver <pool> ...\n"));
 	case HELP_TRIM:
-		return (gettext("\ttrim [-dp] [-r <rate>] [-c | -s] <pool> "
+		return (gettext("\ttrim [-d] [-r <rate>] [-c | -s] <pool> "
 		    "[<device> ...]\n"));
 	case HELP_STATUS:
 		return (gettext("\tstatus [-c [script1,script2,...]] "