Fix a typo in the documentation of expandtabs

markdown.dtx

@@ -15733,7 +15733,7 @@ function M.reader.new(writer, options)
 % \begin{markdown}
 %
 % Define \luamdef{expandtabs} either as an identity function, when the
-% \Opt{preserveTabs} Lua inrerface option is enabled, or to a function that
+% \Opt{preserveTabs} Lua interface option is enabled, or to a function that
 % expands tabs into spaces otherwise.
 %
 % \end{markdown}