Input: fix typos in Documentation/input/gamepad.txt

Fix some typos and while at it also use "PS" as the name for the central
"HOME" button on Sony controllers, this is how Sony itself calls it.

Reviewed-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Antonio Ospite <ospite@studenti.unina.it>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>