Documentation: input: fix path to struct ff_effect's definition

The UAPI header split failed to update the documentation here; fix things
accordingly.

Signed-off-by: Martin Kepplinger <martink@posteo.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/input/ff.txt

@@ -106,9 +106,9 @@ allocate a new effect.
 
 Effects are file descriptor specific.
 
-See <linux/input.h> for a description of the ff_effect struct. You should also
-find help in a few sketches, contained in files shape.fig and interactive.fig.
-You need xfig to visualize these files.
+See <uapi/linux/input.h> for a description of the ff_effect struct. You should
+also find help in a few sketches, contained in files shape.fig and
+interactive.fig. You need xfig to visualize these files.
 
 3.3 Removing an effect from the device
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~