Add the missing closing backticks

Signed-off-by: Jiaxiao Zhou (Mossaka) <[email protected]>

Author: Mossaka

design/mvp/WIT.md

@@ -359,6 +359,7 @@ world union-my-world-b {
 ```
 
 `with` cannot be used to rename IDs, however, so the following world would be invalid:
+
 ```wit
 package local:demo
 
@@ -374,6 +375,8 @@ world invalid-union-world {
     include my-using-a with { a as b }  // invalid: 'a', which is short for 'local:demo/a', is an ID
 }
 
+```
+
 ### A Note on SubTyping
 
 In the future, when `optional` export is supported, the world author may explicitly mark exports as optional to make a component targeting an included World a subtype of the union World.