Bug 1614022 - Update to Learn More string r=pdahiya

Differential Revision: https://phabricator.services.mozilla.com/D62101

--HG--
extra : moz-landing-system : lando

browser/components/newtab/lib/OnboardingMessageProvider.jsm

@@ -78,7 +78,7 @@ const JOIN_CONTENT = {
     text: { string_id: `onboarding-benefit-${id}-text` },
   })),
   learn: {
-    text: { string_id: "onboarding-welcome-learn-more" },
+    text: { string_id: "onboarding-welcome-modal-family-learn-more" },
     url: "https://www.mozilla.org/firefox/accounts/",
   },
   form: {