Remove multiply-defined refs in documentation

Author: Witiko

markdown.dtx

@@ -14528,7 +14528,7 @@ function M.writer.new(options)
 %
 % Parse the \Opt{slice} option and define \luamdef{writer->slice\_begin}
 % \luamdef{writer->slice\_end}, and \luamdef{writer->is\_writing}. The
-% \luamdef{writer->is\_writing} member variable is mutable.
+% \luam{writer->is\_writing} member variable is mutable.
 %
 % \end{markdown}
 %  \begin{macrocode}
@@ -15123,7 +15123,7 @@ function M.writer.new(options)
 %
 % Define \luamdef{writer->active\_headings} as a stack of identifiers
 % of the headings that are currently active. The
-% \luamdef{writer->active\_headings} member variable is mutable.
+% \luam{writer->active\_headings} member variable is mutable.
 %
 % \end{markdown}
 %  \begin{macrocode}