Fix 6348 (#6717)

* Fixup CPP documentation's markdown errors

Note that I couldn't get the ~~~{.cpp} method to work,
so I switched to using ```cpp which did work.

* Fixup C++ docs, typo in repeated code example

Co-authored-by: Paul Harris <paulharris@computer.org>

docs/source/Tutorial.md

@@ -1820,7 +1820,7 @@ Here is a repetition of these lines, to help highlight them more clearly:
 <div class="language-cpp">
   ~~~{.cpp}
     monster_builder.add_equipped_type(Equipment_Weapon); // Union type
-    monster_builder.add_equipped(axe); // Union data
+    monster_builder.add_equipped(axe.Union()); // Union data
   ~~~
 </div>
 <div class="language-java">