Update message to remove duplicate is (#2466)

Author: AdarshKonchady

packages/workbox-core/src/models/messages/messages.ts

@@ -89,7 +89,7 @@ export const messages: MessageMap = {
     return `Two of the entries passed to ` +
       `'workbox-precaching.PrecacheController.addToCacheList()' had the URL ` +
       `${firstEntry._entryId} but different revision details. Workbox is ` +
-      `is unable to cache and version the asset correctly. Please remove one ` +
+      `unable to cache and version the asset correctly. Please remove one ` +
       `of the entries.`;
   },