docs(client-sns): updated sns client readme v3 sample (#3821)

* Updated SNS Client Readme

- Changed from v3 sample from AddPermission to PublishCommand

* Update README.md

Author: sravimohan

clients/client-sns/README.md

@@ -39,16 +39,16 @@ using your favorite package manager:
 
 The AWS SDK is modulized by clients and commands.
 To send a request, you only need to import the `SNSClient` and
-the commands you need, for example `AddPermissionCommand`:
+the commands you need, for example `PublishCommand`:
 
 ```js
 // ES5 example
-const { SNSClient, AddPermissionCommand } = require("@aws-sdk/client-sns");
+const { SNSClient, PublishCommand } = require("@aws-sdk/client-sns");
 ```
 
 ```ts
 // ES6+ example
-import { SNSClient, AddPermissionCommand } from "@aws-sdk/client-sns";
+import { SNSClient, PublishCommand } from "@aws-sdk/client-sns";
 ```
 
 ### Usage
@@ -62,12 +62,18 @@ To send a request, you:
 
 ```js
 // a client can be shared by different commands.
-const client = new SNSClient({ region: "REGION" });
+const client = new SNSClient({ region: "<YOUR_REGION>" });
+
+const message = {
+  hello":"world"
+}
 
 const params = {
-  /** input parameters */
+  Message: JSON.stringify(message),
+  TopicArn: 'arn:aws:sns:<YOUR_REGION>:<YOUR_ACCOUNT_ID>:<YOUR_TOPIC_NAME>'
 };
-const command = new AddPermissionCommand(params);
+
+const command = new PublishCommand(params);
 ```
 
 #### Async/await