QMP: Improve RTC_CHANGE event description

Some people might think that this event is emitted whenever the
time changes, be more specific.
Signed-off-by: NLuiz Capitulino <lcapitulino@redhat.com>

QMP/qmp-events.txt

@@ -41,7 +41,7 @@ Example:
 RTC_CHANGE
 ----------
 
-Emitted when the RTC time changes.
+Emitted when the guest changes the RTC time.
 
 Data: