[Doc-18193][dolphinscheduler-alert-http] Fix incorrect alert param doc (#18194)

Author: SbloodyS

docs/docs/en/guide/alert/http.md

@@ -12,7 +12,7 @@ If you need to use `Http script` for alerting, create an alert instance in the a
 | Body          | The request body of the `Http` request in JSON format, when using `POST` or `PUT` method to alert.  |
 | Content Type  | The content-type of header.                                                                         |
 
-> Alarm message supports variables `$msg`, which can be used in `URL`, `Headers`, and `Body`.
+> Alarm message supports variables `${msg}`, which can be used in `URL`, `Headers`, and `Body`.
 
 ### GET HTTP
 

docs/docs/zh/guide/alert/http.md

@@ -12,7 +12,7 @@
 | 请求体(Body)    | Http`请求的完整请求体，以JSON为格式，非必填                     |
 | Content-Type | 请求体的`Content-Type`，默认为`application/json`       |
 
-> 告警消息，支持变量`$msg`，可在`URL`,`请求头`,`请求体`中使用，非必填。
+> 告警消息，支持变量`${msg}`，可在`URL`,`请求头`,`请求体`中使用，非必填。
 
 ### GET Http告警