Update Sigma CLI validation instructions in CONTRIBUTING.md

Author: thomaspatzke

CONTRIBUTING.md

@@ -32,7 +32,7 @@ git checkout -b your-feature-branch
    python tests/test_rules.py
    ```
 
-   3.3. **Sigma CLI validation** - runs schema and validation checks using [Sigma CLI](https://github.com/SigmaHQ/sigma-cli). Run this if you have `sigma` installed:
+   3.3. **Sigma CLI validation** - runs schema and validation checks using [Sigma CLI](https://github.com/SigmaHQ/sigma-cli). Run this if you have `sigma` with `pySigma-validators-sigmaHQ` installed:
 
    ```bash
    sigma check --fail-on-error --fail-on-issues --validation-config tests/sigma_cli_conf.yml rules/ rules-emerging-threats/ rules-threat-hunting/ rules-compliance/