Fix invalid YAML syntax in workflow example

The usage example in README contained invalid YAML syntax where the
'on:' trigger used '- push:' format which causes workflow failures.

Changed to the correct format 'push:' without the dash prefix.

Fixes #2

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jozefizso

README.md

@@ -12,7 +12,7 @@ This action requires macOS operating system.
 name: PowerPoint Automation
 
 on:
-  - push:
+  push:
 
 jobs:
   powerpoint-tests: