Update hexer.json with new TXT format and proxied flag

Reformatted TXT record entry and added proxied flag.

domains/hexer.json

@@ -9,6 +9,9 @@
       { "priority": 10, "target": "mx1.improvmx.com" },
       { "priority": 20, "target": "mx2.improvmx.com" }
     ],
-    "TXT": ["v=spf1 include:spf.improvmx.com ~all"]
-  }
+    "TXT": [
+        "v=spf1 include:spf.improvmx.com ~all"
+    ]
+  },
+  "proxied": true
 }