Fix incorrect metric desc for bucketRequestsDuration

[krisis]

Community Contribution License

All community contributions in this pull request are licensed to the project maintainers
under the terms of the Apache 2 license.
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contributions to the project maintainers under the Apache 2 license.

Description

Fixes an incorrect metric description.

How to test this PR?

1. Make a few S3 API calls on objects in a bucket.
2. Observe minio_bucket_requests_ttfb_seconds_distribution like so,
   mc admin prometheus metrics ALIAS bucket | grep minio_bucket_requests_ttfb_seconds_distribution

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Optimization (provides speedup with no functional changes)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• Fixes a regression (If yes, please add commit-id or PR # here)
  PR #18632
• Unit tests added/updated
• Internal documentation updated
• Create a documentation update request here

[vadmeste]

LGTM