Missing translation key (#3281)

* missing translation key

* Update src/lang/en.json

Co-authored-by: Frank Elsinga <frank@elsinga.de>

---------

Co-authored-by: Louis Lam <louislam@users.noreply.github.com>
Co-authored-by: Frank Elsinga <frank@elsinga.de>

src/lang/en.json

@@ -751,5 +751,6 @@
     "Badge URL": "Badge URL",
     "Group": "Group",
     "Monitor Group": "Monitor Group",
-    "noGroupMonitorMsg": "Not Available. Create a Group Monitor First."
+    "noGroupMonitorMsg": "Not Available. Create a Group Monitor First.",
+    "Close": "Close"
 }