fix mention text

.github/workflows/list-unhealthy-branches.yml

@@ -40,7 +40,7 @@ jobs:
         status: custom
         payload: |
           {
-            text: '@channel There is some *illegal named branches* on GitHub.',
+            text: '<@channel> There is some *illegal named branches* on GitHub.',
             channel: '#ci',
             attachments: ${{ steps.list-branches.outputs.SLACK_ATTACHMENTS_ILLEGAL }}
           }
@@ -54,7 +54,7 @@ jobs:
         status: custom
         payload: |
           {
-            text: '@channel There is some *illegal named branches* on GitHub.',
+            text: '<@channel> There is some *illegal named branches* on GitHub.',
             channel: '#ci',
             attachments: ${{ steps.list-branches.outputs.SLACK_ATTACHMENTS_INACTIVE }}
           }