docs: Use proper markup for a literal command line option

Signed-off-by: Martin Storsjö <martin@martin.st>
(cherry picked from commit a3a94e1498)

doc/general.texi

@@ -986,7 +986,7 @@ extern __declspec(dllimport) const AVPixFmtDescriptor av_pix_fmt_descriptors[];
 
 Note that using import libraries created by dlltool requires
 the linker optimization option to be set to
-"References: Keep Unreferenced Data (/OPT:NOREF)", otherwise
+"References: Keep Unreferenced Data (@code{/OPT:NOREF})", otherwise
 the resulting binaries will fail during runtime. This isn't
 required when using import libraries generated by lib.exe.