fix(captions): fix grammar in captions check incomplete message (#4661)

Thanks @somaalapati for reporting the issue

Author: dbjorge

lib/checks/media/caption.json

@@ -5,7 +5,7 @@
     "impact": "critical",
     "messages": {
       "pass": "The multimedia element has a captions track",
-      "incomplete": "Check that captions is available for the element"
+      "incomplete": "Check that captions are available for the element"
     }
   }
 }

locales/_template.json

@@ -845,7 +845,7 @@
     },
     "caption": {
       "pass": "The multimedia element has a captions track",
-      "incomplete": "Check that captions is available for the element"
+      "incomplete": "Check that captions are available for the element"
     },
     "frame-tested": {
       "pass": "The iframe was tested with axe-core",