fix: update Grafana dashboard to reflect `throttle` metrics rename

[squat]

The metrics for throttling reasons have been renamed from nvidia_smi_clocks_throttle_reasons_* to nvidia_smi_clocks_event_reasons_*. This change has broken the Grafana dashboard.

xref: https://github.com/utkuozdemir/nvidia_gpu_exporter/issues/123

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [x] All new and existing tests passed.