doc: update libavfilter documentation

Update the reference to the conversion tool to use avconv and
make sure the example line works as supposed.
Remove the paragraph pointing to the svn repo

doc/libavfilter.texi

@@ -14,18 +14,6 @@
 Libavfilter is the filtering API of Libav. It is the substitute of the
 now deprecated 'vhooks' and started as a Google Summer of Code project.
 
-Integrating libavfilter into the main Libav repository is a work in
-progress. If you wish to try the unfinished development code of
-libavfilter then check it out from the libavfilter repository into
-some directory of your choice by:
-
-@example
-   svn checkout svn://svn.libav.org/soc/libavfilter
-@end example
-
-And then read the README file in the top directory to learn how to
-integrate it into ffmpeg and avplay.
-
 But note that there may still be serious bugs in the code and its API
 and ABI should not be considered stable yet!
 
@@ -48,15 +36,14 @@ and the vflip filter before merging it back with the other stream by
 overlaying it on top. You can use the following command to achieve this:
 
 @example
-./ffmpeg -i in.avi -s 240x320 -vf "[in] split [T1], fifo, [T2] overlay= 0:240 [out]; [T1] fifo, crop=0:0:-1:240, vflip [T2]
+./avconv -i input -vf "[in] split [T1], fifo, [T2] overlay=0:H/2 [out]; [T1] fifo, crop=iw:ih/2:0:ih/2, vflip [T2]" output
 @end example
 
-where input_video.avi has a vertical resolution of 480 pixels. The
-result will be that in output the top half of the video is mirrored
+The result will be that in output the top half of the video is mirrored
 onto the bottom half.
 
 Video filters are loaded using the @var{-vf} option passed to
-ffmpeg or to avplay. Filters in the same linear chain are separated by
+avconv or to avplay. Filters in the same linear chain are separated by
 commas. In our example, @var{split, fifo, overlay} are in one linear
 chain, and @var{fifo, crop, vflip} are in another. The points where
 the linear chains join are labeled by names enclosed in square