target/target: Fix 'wp' command usage

While at it, fix the 'wp' command documentation.

Change-Id: I70f3110e8ce286051f8f810260f1857b2285e634
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8022
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Jan Matyas <jan.matyas@codasip.com>

src/target/target.c

@@ -7064,7 +7064,7 @@ static const struct command_registration target_exec_command_handlers[] = {
 		.handler = handle_wp_command,
 		.mode = COMMAND_EXEC,
 		.help = "list (no params) or create watchpoints",
-		.usage = "[address length [('r'|'w'|'a') value [mask]]]",
+		.usage = "[address length [('r'|'w'|'a') [value [mask]]]]",
 	},
 	{
 		.name = "rwp",