docs: update GitHub markdown syntax

Looks like GitHub deprecated the old syntax. New syntax gleaned from
https://github.com/orgs/community/discussions/16925

Author: EricCrosson

README.md

@@ -37,7 +37,7 @@ Use this action in a step:
     targets: EricCrosson/flux-capacitor@v1
 ```
 
-> **Note**
+> [!NOTE]
 > I recommend adding an explicit step name, otherwise the step will only reference
 > `EricCrosson/install-github-release-binary@v2`, not your targets.