docs: Tell pandoc we need Docbook4

gtk-doc assumes Docbook4, with <ulink> and so on.
Without this, all the links in markdown are converted
to <link xlink:href=...> and then lost in the docbook->html
conversion.

docs/reference/gtk/gtk-markdown-to-docbook

@@ -180,7 +180,7 @@ def ConvertToDocbook(infile, outfile):
     else:
         division='chapter'
     input_format = "markdown" + "".join(MarkdownExtensions)
-    output_format = "docbook"
+    output_format = "docbook4"
     subprocess.check_call(["pandoc", infile, "-o", outfile,
                            "--from=" + input_format,
                            "--to=" + output_format,