2nd try on documenting av_gcd().

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@20939 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavutil/mathematics.h

@@ -58,7 +58,7 @@ enum AVRounding {
 
 /**
  * Returns the greatest common divisor of a and b.
- * If either a or b are 0 or either or both are <0 then behavior is
+ * If both a and b are 0 or either or both are <0 then behavior is
  * undefined.
  */
 int64_t av_const av_gcd(int64_t a, int64_t b);