2nd try on documenting av_gcd().

Originally committed as revision 20939 to svn://svn.ffmpeg.org/ffmpeg/trunk

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);