Capitalize href (#998)

Author: mycho2003

api-playground/openapi-setup.mdx

@@ -154,7 +154,7 @@ Add content before the auto-generated API documentation using `x-mint: content`:
 
 The `content` extension supports all Mintlify MDX components and formatting.
 
-### href
+### Href
 
 Change the URL of the endpoint page in your docs using `x-mint: href`: