Update README.md

Author: vimalloc

README.md

@@ -16,7 +16,7 @@ easier. These include:
 ### Usage
 [View the documentation online](https://flask-jwt-extended.readthedocs.io/en/stable/)
 
-## Upgrading from 3.x.x to 4.0.0
+### Upgrading from 3.x.x to 4.0.0
 [View the changes](https://flask-jwt-extended.readthedocs.io/en/stable/v4_upgrade_guide/)
 
 ### Changelog