doc/demuxers: quote command line arguments with @code

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

doc/demuxers.texi

@@ -6,14 +6,14 @@ multimedia streams from a particular type of file.
 
 When you configure your FFmpeg build, all the supported demuxers
 are enabled by default. You can list all available ones using the
-configure option "--list-demuxers".
+configure option @code{--list-demuxers}.
 
 You can disable all the demuxers using the configure option
-"--disable-demuxers", and selectively enable a single demuxer with
-the option "--enable-demuxer=@var{DEMUXER}", or disable it
-with the option "--disable-demuxer=@var{DEMUXER}".
+@code{--disable-demuxers}, and selectively enable a single demuxer with
+the option @code{--enable-demuxer=@var{DEMUXER}}, or disable it
+with the option @code{--disable-demuxer=@var{DEMUXER}}.
 
-The option "-formats" of the ff* tools will display the list of
+The option @code{-formats} of the ff* tools will display the list of
 enabled demuxers.
 
 The description of some of the currently available demuxers follows.