doc/muxers: fix level for the segment options subsection

Properly declare it like a @subsection, rather than as a @section.

doc/muxers.texi

@@ -645,7 +645,7 @@ of the generated segments. May not work with some combinations of
 muxers/codecs. It is set to @code{0} by default.
 @end table
 
-@section Examples
+@subsection Examples
 
 @itemize
 @item