doc: Fit the graph in 80 cols

Some man pagers do not reflow the text/graphs on resize
and the graphs did not fit the traditional 80cols due
the pod2man excessive indent.
Merge commit '006c253346d90f4abaf69d78083c8ba031628598'

* commit '006c253346d90f4abaf69d78083c8ba031628598':
  doc: Fit the graph in 80 cols

Merged-by: Michael Niedermayer <michaelni@gmx.at>
doc/ffmpeg: restore ascii arrows in Simple filtergraphs

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>