powerpc/xmon/sysrq: fix inconstistent help message of sysrq key

Currently help message of /proc/sysrq-trigger highlight its
upper-case characters, like below:

      SysRq : HELP : loglevel(0-9) reBoot Crash terminate-all-tasks(E)
      memory-full-oom-kill(F) kill-all-tasks(I) ...

this would confuse user trigger sysrq by upper-case character, which is
inconsistent with the real lower-case character registed key.

This inconsistent help message will also lead more confused when
26 upper-case letters put into use in future.

This patch fix powerpc xmon sysrq key: "xmon(x)"
Signed-off-by: Nzhangwei(Jovi) <jovi.zhangwei@huawei.com>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

arch/powerpc/xmon/xmon.c

@@ -2947,7 +2947,7 @@ static void sysrq_handle_xmon(int key)
 
 static struct sysrq_key_op sysrq_xmon_op = {
 	.handler =	sysrq_handle_xmon,
-	.help_msg =	"Xmon",
+	.help_msg =	"xmon(x)",
 	.action_msg =	"Entering xmon",
 };