Add missing period in av_fill_image_max_pixstep() doxy.

Originally committed as revision 24850 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcore/imgutils.h

@@ -29,7 +29,7 @@
 
 /**
  * Compute the max pixel step for each plane of an image with a
- * format described by pixdesc
+ * format described by pixdesc.
  *
  * The pixel step is the distance in bytes between the first byte of
  * the group of bytes which describe a pixel component and the first