examples/encoding.c: replace "this library" with "libavcodec" in doxy

So it is more clear the context of the sentence.

doc/examples/encoding.c

@@ -22,7 +22,7 @@
  * @file
  * libavcodec API use example.
  *
- * Note that this library only handles codecs (mpeg, mpeg4, etc...),
+ * Note that libavcodec only handles codecs (mpeg, mpeg4, etc...),
  * not file formats (avi, vob, etc...). See library 'libavformat' for the
  * format handling
  */