You said you wanted to display the markdown as-is, that’s why I provided you with instructions to display the markdown as-is and not rendered. If you want to display it rendered as HTML, you could try replacing
@Inject
@Named("plain/1.0")
private Parser plainTextParser;
by
@Inject
@Named("markdown+github/1.0")
private Parser plainTextParser;
Of course you could/should also rename the variable to something like markdownParser to make the code clearer. If you’re using a recent-enough XWiki version (14.10.5/15.1), you could also use a MacroContentParser which provides some more convenience features. Further, if you’re on 14.2 or more recent, you could also pass an id generator to the parse method to ensure that ids, e.g., of headings, are unique across the whole document. You can get the id generator using macroContext.getXDOM().getIdGenerator() where macroContext is the context parameter of type MacroTransformationContext that is passed to the execute-method of the macro.
The MacroBlock for the content macro might work, too, though you should definitely set the inline parameter to false if this should be a whole document.