docs(customl-layout): append swagger-ui css class (#9329)

To display style properly, we need to add className="swagger-ui"
to the layout.

docs/customization/custom-layout.md

@@ -21,7 +21,7 @@ class OperationsLayout extends React.Component {
     const Operations = getComponent("operations", true)
 
     return (
-      <div>
+      <div className="swagger-ui">
         <Operations />
       </div>
     )