Update properties to document lastUpdated

Author: cassierecher

google-codelab.html

@@ -86,7 +86,7 @@
 
         <template is="dom-if" if="{{lastUpdated}}" strip-whitespace>
           <div id="last-updated">
-            Last updated on <span>{{lastUpdated}}</span>
+            Last updated on {{lastUpdated}}.
           </div>
         </template>
 
@@ -237,6 +237,15 @@ <h2>Would you like to resume where you left off?</h2>
           readOnly: true
         },
 
+        /**
+         * Last date the codelab was updated, in YYYY-MM-DD format.
+         */
+        lastUpdated: {
+          type: String,
+          value: '',
+          readOnly: true
+        },
+
         /**
          * Step elements of the codelab. Currently, only `<google-codelab-step>`
          * elements are supported.