typo fixed

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@8130 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

doc/snow.txt

@@ -162,7 +162,7 @@ qlog
 
 mv_scale
     stored as delta from last, last is reset to 0 if always_reset || keyframe
-    FIXME check that everything works fine if this chanes between frames
+    FIXME check that everything works fine if this changes between frames
 
 qbias
     dequantization bias