Fix the grype ruletype validation (#276)

Signed-off-by: Radoslav Dimitrov <[email protected]>

rule-types/github/grype_github_action_scan_container_image.yaml

@@ -23,7 +23,7 @@ guidance: |
   - name: Scan image
     uses: anchore/[email protected]
     with:
-      image: "<image-reference>"
+      image: "your-image-reference"
   ```
 
   For more details, refer to the [Grype action documentation](https://github.com/anchore/scan-action).