doc: fix a couple of typos in frame.h

libavutil/frame.h

@@ -428,7 +428,7 @@ AVFrame *av_frame_alloc(void);
 void av_frame_free(AVFrame **frame);
 
 /**
- * Setup a new reference to the data described by an given frame.
+ * Set up a new reference to the data described by the source frame.
  *
  * Copy frame properties from src to dst and create a new reference for each
  * AVBufferRef from src.