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 <[e-mail]>
Signed-off-by: Dmitry Torokhov <[e-mail]>

Documentation/input/ff.rst

···
 
 Before you start, let me WARN you that some devices shake violently during the
 initialisation phase. This happens for example with my "AVB Top Shot Pegasus".
-To stop this annoying behaviour, move you joystick to its limits. Anyway, you
+To stop this annoying behaviour, move your joystick to its limits. Anyway, you
 should keep a hand on your device, in order to avoid it to break down if
 something goes wrong.
 
···
 The content of effect may be modified. In particular, its field "id" is set
 to the unique id assigned by the driver. This data is required for performing
 some operations (removing an effect, controlling the playback).
-This if field must be set to -1 by the user in order to tell the driver to
+The "id" field must be set to -1 by the user in order to tell the driver to
 allocate a new effect.
 
 Effects are file descriptor specific.
···
 	stop.code = effect.id;
 	stop.value = 0;
 
-	write(fd, (const void*) &play, sizeof(stop));
+	write(fd, (const void*) &stop, sizeof(stop));
 
 Setting the gain
 ----------------