Input: fix typo in documentation

Event type for key presses is EV_KEY, not REL_KEY.
Signed-off-by: Dmitry Torokhov <dtor@mail.ru>

Documentation/input/input.txt

@@ -278,7 +278,7 @@ struct input_event {
 };
 
   'time' is the timestamp, it returns the time at which the event happened.
-Type is for example EV_REL for relative moment, REL_KEY for a keypress or
+Type is for example EV_REL for relative moment, EV_KEY for a keypress or
 release. More types are defined in include/linux/input.h.
 
   'code' is event code, for example REL_X or KEY_BACKSPACE, again a complete