Fix to clarify error

docs/docs/what-is-mdx.mdx

@@ -318,9 +318,9 @@ So you *can* build incorrect HTML (which you shouldn’t):
 ```
 
 It’s not possible to wrap “blocks” if text and tags are on the same line but the
-corresponding tags are on different lines:
+corresponding tags are on different lines (so this is invalid!):
 
-```mdx
+```
 Welcome! <a href="about.html">
 
 This is home of...
@@ -332,6 +332,7 @@ That’s because to parse markdown, we first have to divide it into “blocks”
 So in this case two paragraphs and a heading.
 Leaving an opening `a` tag in the first paragraph and a stray closing `a` tag in
 the heading.
+Which causes an error.
 
 ## Further reading