shin.json: change record MX and TXT

change service fordward email from `improvmx` to `forwardemail`

domains/shin.json

@@ -14,9 +14,9 @@
         "185.199.111.153"
     ],
     "MX": [
-        "mx1.improvmx.com",
-        "mx2.improvmx.com"
+        "mx1.forwardemail.net",
+        "mx2.forwardemail.net"
     ],
-    "TXT": "v=spf1 include:spf.improvmx.com ~all"
+    "TXT": "forward-email=tachibshin@duck.com"
   }
 }