Fix invalid reference format: repository name must be lowercase error

ref a7edf7e#r139948703

Author: ohbarye

action.yml

@@ -17,13 +17,13 @@ runs:
           SLACK_MESSAGE: "${{ steps.slackify.outputs.text }}"
           GITHUB_RUN: "${{ github.event.repository.html_url }}/actions/runs/${{ github.run_id }}"
           ENABLE_ESCAPES: "true"
-        uses: "docker://ghcr.io/rtCamp/action-slack-notify:v2.2.1"
+        uses: "docker://ghcr.io/rtcamp/action-slack-notify:v2.2.1"
 
       - name: "Slack Notification (Unformatted)"
         if: env.SLACKIFY_MARKDOWN != 'true'
-        uses: "docker://ghcr.io/rtCamp/action-slack-notify:v2.2.1"
+        uses: "docker://ghcr.io/rtcamp/action-slack-notify:v2.2.1"
         env:
           GITHUB_RUN: "${{ github.event.repository.html_url }}/actions/runs/${{ github.run_id }}"
 branding:
   icon: 'bell'
-  color: 'yellow'
+  color: 'yellow'