Bug 1546093 - Fixing typo of about-telemetry-time-stamp-header leading to missing translation in en-US r=Gijs

Differential Revision: https://phabricator.services.mozilla.com/D28734

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

toolkit/content/aboutTelemetry.js

@@ -1817,7 +1817,7 @@ var Events = {
     setHasData("events-section", hasData);
     if (Object.keys(events).length > 0) {
       const headings = [
-        "about-telemetry-timestamp-header",
+        "about-telemetry-time-stamp-header",
         "about-telemetry-category-header",
         "about-telemetry-method-header",
         "about-telemetry-object-header",