Correct loopback example

"tag" was written instead of "inboundTag" in the example which caused much frustration for me when I was trying to configure my server using the example provided.
I corrected it to spare others from having the same problem.

docs/en/config/outbounds/loopback.md

@@ -27,7 +27,7 @@ If you need to do some more detailed routing for traffics that have been routed
       "protocol": "loopback",
       "tag": "need-to-split",
       "settings": {
-        "tag": "traffic-input" // This tag will be used as the inboundTag inside the RuleObject 
+        "inboundTag": "traffic-input" // This tag will be used as the inboundTag inside the RuleObject 
       }
     },
     {
@@ -54,4 +54,4 @@ If you need to do some more detailed routing for traffics that have been routed
     ]
   }
 }
-```
+```