doc: replace avserver by ffserver as we have not done that rename

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

doc/avconv.texi

@@ -198,7 +198,7 @@ Set the number of video frames to record. This is an alias for @code{-frames:v}.
 @item -r[:@var{stream_specifier}] @var{fps} (@emph{input/output,per-stream})
 Set frame rate (Hz value, fraction or abbreviation), (default = 25).
 @item -s[:@var{stream_specifier}] @var{size} (@emph{input/output,per-stream})
-Set frame size. The format is @samp{wxh} (avserver default = 160x128, avconv default = same as source).
+Set frame size. The format is @samp{wxh} (ffserver default = 160x128, avconv default = same as source).
 The following abbreviations are recognized:
 @table @samp
 @item sqcif