fix: wrong parameter name in test_multi_iterations; clarify timezone docs

[alubbock]

Summary

• test_multi_iterations passed timezone= to MicroBench.__init__, but the parameter is tz=. The timezone was silently stored as a static metadata field instead of being applied, so timestamps remained in UTC. The test then checked results['timezone'] (the static field) rather than results['timestamp_tz'] (the actual timezone label).
• Fixed to use tz= and assert against results['timestamp_tz']
• Expanded the Timezones section of the README to clarify that timestamp_tz records the timezone for reference, and shows how to use the local machine's timezone