message: fix MenuItem example in README
diff: @@ -188,8 +188,8 @@ class CustomGraphiQL extends React.Component {
  />
  // Some other possible toolbar items
  - <GraphiQL.Menu title="File">
  - <GraphiQL.MenuItem title="Save" onClick={...}>
  + <GraphiQL.Menu label="File" title="File">
  + <GraphiQL.MenuItem label="Save" title="Save" onSelect={...}>
  </GraphiQL.Menu>
  <OtherReactComponent someProps="true" />
  