fix small typo introduced in https://trac.osgeo.org/grass/changeset/63551

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@63552 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/pngdriver/raster.c

@@ -75,7 +75,7 @@ static void alloc_buffers(void)
 
   \param mask non-zero int for mask
   \param s source (map) extent (left, right, top, bottom)
-  \param d destination (image) extent (left, right, top, bottom)
+  \param fd destination (image) extent (left, right, top, bottom)
 */
 void PNG_begin_raster(int mask, int s[2][2], double fd[2][2])
 {