Update bad ffmpeg compile flag in user manual: --enable-x11grab to --enable-libxcb

[jcahill]

User manual: installing MediacenterJS's ffmpeg instructions don't pass anymore (deprecated).

Compile FFMPEG for Linux

./configure --prefix="$HOME/ffmpeg_build" \
   --extra-cflags="-I$HOME/ffmpeg_build/include" --extra-ldflags="-L$HOME/ffmpeg_build/lib" \
   --bindir="$HOME/bin" --extra-libs="-ldl" --enable-gpl --enable-libass \
   --enable-libmp3lame --enable-libtheora --enable-libvorbis \
   --enable-libx264 --enable-nonfree --enable-x11grab

now needs to be:

./configure --prefix="$HOME/ffmpeg_build" \
   --extra-cflags="-I$HOME/ffmpeg_build/include" --extra-ldflags="-L$HOME/ffmpeg_build/lib" \
   --bindir="$HOME/bin" --extra-libs="-ldl" --enable-gpl --enable-libass \
   --enable-libmp3lame --enable-libtheora --enable-libvorbis \
   --enable-libx264 --enable-nonfree --enable-libxcb

[jansmolders86]

Thanks, changed the doc!