Fix incorrect heading level (#176)

Change 'Adding year groups to schools already in Mavis' from H3 to H2.

Author: melissagraham-create

app/guide/managing-schools.md

@@ -77,7 +77,7 @@ You can edit a site’s name and/or address, or the year groups it serves.
 4. Make any changes you need to make then select **Continue**.
 5. Review the updated information and select **Save changes**.
 
-### Adding year groups to schools already in Mavis
+## Adding year groups to schools already in Mavis
 
 If a school does not include the correct school years, you can add them.