docs: normalise table headers

Those jobs trigger a Gitleaks analysis (either on the complete repository, either on the current branch).

They use the following configuration.

| Name                  | description                            | default value     |

| Name                  | Description                            | Default value     |

| --------------------- | -------------------------------------- | ----------------- |

| `GITLEAKS_IMAGE`      | The Docker image used to run Gitleaks  | `registry.hub.docker.com/zricethezav/gitleaks:latest` |

| `GITLEAKS_RULES`      | Gitleaks [configuration rules](https://github.com/zricethezav/gitleaks#configuration) to use (you may also provide your own `.gitleaks.toml` configuration file in your project). | _none_ (uses default rules) |