Input: fix small typos in force feedback documentation

Fix small typos in the Instructions and Uploading sections. Fix a typo in
the start/stop effect example usage code.

Signed-off-by: Jean-François Têtu <jean-francois.tetu@savoirfairelinux.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>