add link to hub submodule intro guide

Author: jyaistMap

guide/02-api-overview/release_notes_220.ipynb

@@ -24,7 +24,7 @@
     "* [Administration](../)\n",
     "  * [Managing ArcGIS Applications](../)\n",
     "    * [Working with ArcGIS Hub](../)\n",
-    "      * [The Hub Submodule]()\n",
+    "      * [The Hub Submodule](../guide/hub-submodule-intro)\n",
     "      * [Introduction to ArcGIS Hub Basic](../guide/hub-basic-guide/)\n",
     "      * [Introduction to ArcGIS Hub Premium](../guide/hub-premium-guide/)\n",
     "      * [Introduction to ArcGIS Enterprise Sites](../guide/enterprise-sites-guide/)\n",