README: fix typo and description of libavfilter

Thanks to Arif Driessen for bringing notice to the typo.

README.md

@@ -9,7 +9,7 @@ such as audio, video, subtitles and related metadata.
 * `libavcodec` provides implementation of a wider range of codecs.
 * `libavformat` implements streaming protocols, container formats and basic I/O access.
 * `libavutil` includes hashers, decompressors and miscellaneous utility functions.
-* `libavfilter` provides a mean to alter decoded Audio and Video through chain of filters.
+* `libavfilter` provides means to alter decoded audio and video through a directed graph of connected filters.
 * `libavdevice` provides an abstraction to access capture and playback devices.
 * `libswresample` implements audio mixing and resampling routines.
 * `libswscale` implements color conversion and scaling routines.