fix Missing 404 swagger response docs for /admin/users/{username} (go-gitea#26086)

close go-gitea#26079

Author: CaiCandong

routers/api/v1/admin/user.go

@@ -312,6 +312,8 @@ func DeleteUser(ctx *context.APIContext) {
 	//     "$ref": "#/responses/empty"
 	//   "403":
 	//     "$ref": "#/responses/forbidden"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
 	//   "422":
 	//     "$ref": "#/responses/validationError"