swscale/slice: Fix doc typo

Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libswscale/slice.c

@@ -39,7 +39,7 @@ static void free_lines(SwsSlice *s)
 }
 
 /*
- slice lines contains extra bytes for vetorial code thus @size
+ slice lines contains extra bytes for vectorial code thus @size
  is the allocated memory size and @width is the number of pixels
 */
 static int alloc_lines(SwsSlice *s, int size, int width)