doc/muxers.texi: fix build issue for unknown command

The build log:
** Unknown command `@code' (left as is) (in src/doc/muxers.texi l. 2020)
*** '{' without macro. Before: -map} option with the ffmpeg CLI tool. (in src/doc/muxers.texi l. 2020)
*** '}' without opening '{' before:  option with the ffmpeg CLI tool. (in src/doc/muxers.texi l. 2020)

doc/muxers.texi

@@ -2017,7 +2017,7 @@ No extension is registered so format name has to be supplied e.g. with the ffmpe
 @subsection data
 
 Data muxer accepts a single stream with any codec of any type.
-The input stream has to be selected using the @code {-map} option with the ffmpeg CLI tool.
+The input stream has to be selected using the @code{-map} option with the ffmpeg CLI tool.
 
 No extension is registered so format name has to be supplied e.g. with the ffmpeg CLI tool @code{-f data}.