Fix selectClasspathRoots() example in LauncherDiscoveryRequestBuilder

DiscoverySelectors.selectClasspathRoots() returns a List and therefore
cannot be used alongside any other selectors.

Author: sbrannen

junit-platform-launcher/src/main/java/org/junit/platform/launcher/core/LauncherDiscoveryRequestBuilder.java

@@ -65,10 +65,12 @@
  *        selectMethod("org.example.order.OrderTests", "test4"),
  *        selectMethod(OrderTests.class, "test5"),
  *        selectMethod(OrderTests.class, testMethod),
- *        selectClasspathRoots(Collections.singleton(Paths.get("/my/local/path1"))),
  *        selectUniqueId("unique-id-1"),
  *        selectUniqueId("unique-id-2")
  *     )
+ *     .selectors(
+ *        selectClasspathRoots(Set.of(Paths.get("/my/local/path1")))
+ *     )
  *     .filters(
  *        includeEngines("junit-jupiter", "spek"),
  *        // excludeEngines("junit-vintage"),