Update README.md on how to disable secret-detection job

This merge request updates the README.md file on how to disable the secret_detection job using SECRET_DETECTION_DISABLED.

Below are two examples from the demo project in which only the new snippet works.

• Old Snippet: gitlab-org/secure/tests/ci-components/test-secret-detection-component@f9eda4f1 (check pipelines tab)
• New Snippet: gitlab-org/secure/tests/ci-components/test-secret-detection-component@e918d8c9 (check pipelines tab)

Please note: 1 is also used together with true to keep in line with the corresponding CI template (see this line).