Fix grammar for avfilter_graph_parse() doxy.

Originally committed as revision 18900 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavfilter/graphparser.h

@@ -37,7 +37,7 @@ typedef struct AVFilterInOut {
 } AVFilterInOut;
 
 /**
- * Add to a graph a graph described by a string.
+ * Adds a graph described by a string to a graph.
  *
  * @param graph   the filter graph where to link the parsed graph context
  * @param filters string to be parsed