Input: fix a double word "is is" in include/linux/input.h

This patch fix a double word "is is" found in in
Documentation/DocBook/device-drivers.xml.
It is because the file was created from comments in sources,
so I have to fix the double words in include/linux/input.h

Signed-off-by: Masanari Iida <standby24x7@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>