Align user name vs username (#27328)

Align to be Username as all other integrations

homeassistant/components/transmission/strings.json

@@ -7,7 +7,7 @@
                 "data": {
                     "name": "Name",
                     "host": "Host",
-                    "username": "User name",
+                    "username": "Username",
                     "password": "Password",
                     "port": "Port"
                 }
@@ -37,4 +37,4 @@
             }
         }
     }
-}
+}