doc/avconv: -sample_fmts, not -help sample_fmts prints the sample formats

doc/avconv.texi

@@ -549,7 +549,7 @@ Disable audio recording.
 @item -acodec @var{codec} (@emph{input/output})
 Set the audio codec. This is an alias for @code{-codec:a}.
 @item -sample_fmt[:@var{stream_specifier}] @var{sample_fmt} (@emph{output,per-stream})
-Set the audio sample format. Use @code{-help sample_fmts} to get a list
+Set the audio sample format. Use @code{-sample_fmts} to get a list
 of supported sample formats.
 @end table