doc/writing_filters: fix reference to Lenna image

doc/writing_filters.txt

@@ -23,7 +23,8 @@ for others using the `./ffmpeg -v 0 -filters|grep ' V->V '` command.
    pattern of the other filters.
  - ./configure ...
  - make -j<whatever> ffmpeg
- - ./ffmpeg -i tests/lena.pnm -vf foobar foobar.png
+ - ./ffmpeg -i http://upload.wikimedia.org/wikipedia/en/2/24/Lenna.png -vf foobar foobar.png
+   Note here: you can obviously use a random local image instead of a remote URL.
 
 If everything went right, you should get a foobar.png with Lena edge-detected.