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

Clarify that the 'cat' command does not include the (c, 13, 32)
after it.

Reported-by: Dan Jidanni Jacobson <jidanni@jidanni.org>
Signed-off-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>