Fix a typo in SDL vout plugin

It was preventing mouse to work properly.
Signed-off-by: Christophe Mutricy <xtophe@videolan.org>
(cherry picked from commit 31412310)

modules/video_output/sdl.c

@@ -464,7 +464,7 @@ static int Manage( vout_thread_t *p_vout )
 
                     var_Get( p_vout, "mouse-button-down", &val );
                     val.i_int &= ~4;
-                    var_Set( p_vout, "mous-button-down", val );
+                    var_Set( p_vout, "mouse-button-down", val );
                     p_intf = vlc_object_find( p_vout, VLC_OBJECT_INTF,
                                                       FIND_ANYWHERE );
                     if( p_intf )