docs: fix channel mention formatting

Author: mwbrooks

docs/sending-techniques/sending-data-slack-api-method/sending-data-slack-api-method.md

@@ -35,7 +35,7 @@ Posting a message with the [`chat.postMessage`](https://docs.slack.dev/reference
     token: ${{ secrets.SLACK_BOT_TOKEN }}
     payload: |
       channel: ${{ secrets.SLACK_CHANNEL_ID }}
-      text: "howdy <@channel>!"
+      text: "howdy <!channel>!"
 ```
 
 ### Posting a message with blocks
@@ -142,4 +142,4 @@ Calling [a Slack API method](https://docs.slack.dev/reference/methods) with [`@s
 ## Example workflows
 
 * [**Direct message the author**](/slack-github-action/sending-techniques/sending-data-slack-api-method/direct-message-author): Write to the Slack user with a matching email.
-* [**Invite a usergroup to channel**](/slack-github-action/sending-techniques/sending-data-slack-api-method/invite-usergroup-to-channel): Create a channel and invite members.
+* [**Invite a usergroup to channel**](/slack-github-action/sending-techniques/sending-data-slack-api-method/invite-usergroup-to-channel): Create a channel and invite members.