Fix wrong import in README.md

Replace `identityApi` with `identityApiRef` when importing from
@backstage/core-plugin-api.

Signed-off-by: Clemens Stefan Heithecker <48448358+clemensheithecker@users.noreply.github.com>
Signed-off-by: Clemens Stefan Heithecker <clemens@heithecker.dev>

plugins/user-settings-backend/README.md

@@ -54,7 +54,7 @@ To make use of the user settings backend, replace the `WebStorage` with the
 +  discoveryApiRef,
 +  fetchApiRef,
    errorApiRef,
-+  identityApi,
++  identityApiRef,
 +  storageApiRef,
  } from '@backstage/core-plugin-api';
 +import { UserSettingsStorage } from '@backstage/plugin-user-settings';