Fix errors in Navbar tag type

We were trying to use "tag" instead of "this.tag" and expecting
it to be a String instead of an Object

src/components/PreviousNext.vue

@@ -11,7 +11,7 @@
           <header class="read-next-card-header">
             <small class="read-next-card-header-sitetitle">&mdash; {{ Admin.site.title }} &mdash;</small>
             <h3 class="read-next-card-header-title">
-              <a :href="tag.path">{{ tag.title.replace('-', ' ') | capitalizeFilter }}</a>
+              <a :href="this.tag.path">{{ this.tag.title.replace('-', ' ') | capitalizeFilter }}</a>
             </h3>
           </header>
           <div class="read-next-divider">
@@ -60,7 +60,7 @@ export default {
       type: String
     },
     tag: {
-      type: String
+      type: Object
 		},
 		posts: {
 			type: Array