Update the missing build id message (#4780)

Author: mrober

firebase-crashlytics/src/main/java/com/google/firebase/crashlytics/internal/common/CrashlyticsCore.java

@@ -47,10 +47,10 @@
 @SuppressWarnings("PMD.NullAssignment")
 public class CrashlyticsCore {
   private static final String MISSING_BUILD_ID_MSG =
-      "The Crashlytics build ID is missing. This "
-          + "occurs when Crashlytics tooling is absent from your app's build configuration. "
-          + "Please review Crashlytics onboarding instructions and ensure you have a valid "
-          + "Crashlytics account.";
+      "The Crashlytics build ID is missing. This occurs when the Crashlytics Gradle plugin is "
+          + "missing from your app's build configuration. Please review the Firebase Crashlytics "
+          + "onboarding instructions at "
+          + "https://firebase.google.com/docs/crashlytics/get-started?platform=android#add-plugin";
 
   static final int MAX_STACK_SIZE = 1024;
   static final int NUM_STACK_REPETITIONS_ALLOWED = 10;