Bug 1690916, use sentence case for learn more link, r=fluent-reviewers,Gijs

Differential Revision: https://phabricator.services.mozilla.com/D110874

browser/locales/en-US/browser/browser.ftl

@@ -568,7 +568,7 @@ pointerlock-warning-no-domain = This document has control of your pointer. Press
 
 crashed-subframe-message = <strong>Part of this page crashed.</strong> To let { -brand-product-name } know about this issue and get it fixed faster, please submit a report.
 crashed-subframe-learnmore-link =
-  .value = Learn More
+  .value = Learn more
 crashed-subframe-submit =
   .label = Submit report
   .accesskey = S