Documentation: input.txt: clarify mousedev 'cat' command syntax

Clarify that the 'cat' command does not include the (c, 13, 32)
after it.
Reported-by: NDan Jidanni Jacobson <jidanni@jidanni.org>
Signed-off-by: NRandy Dunlap <rdunlap@xenotime.net>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Documentation/input/input.txt

@@ -250,8 +250,8 @@ And so on up to event31.
 a USB keyboard works and is correctly connected to the kernel keyboard
 driver. 
 
-  Doing a cat /dev/input/mouse0 (c, 13, 32) will verify that a mouse
-is also emulated, characters should appear if you move it.
+  Doing a "cat /dev/input/mouse0" (c, 13, 32) will verify that a mouse
+is also emulated; characters should appear if you move it.
 
   You can test the joystick emulation with the 'jstest' utility,
 available in the joystick package (see Documentation/input/joystick.txt).