nixos/make-options-doc: improve CommonMark formatting

Render the `type` attribute in a code block to match the rest of the
attributes.

nixos/lib/make-options-doc/generateDoc.py

@@ -40,8 +40,10 @@ def generate_commonmark(options):
         print(value['description'])
         print()
         if 'type' in value:
-            print('*_Type_*:')
+            print('*_Type_*')
+            print ('```')
             print(value['type'])
+            print ('```')
         print()
         print()
         if 'default' in value: