Fix markdown links in openelemetry-etw-metrics README

[mattbodd]

Fixes # Design discussion issue (if applicable) #

Changes

Please provide a brief description of the changes here.

Merge requirement checklist

• [ ] CONTRIBUTING guidelines followed
• [ ] Unit tests added/updated (if applicable)
• [ ] Appropriate CHANGELOG.md files updated for non-trivial, user-facing changes
• [ ] Changes in public API reviewed (if applicable)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 50.3%. Comparing base (4ef9bcd) to head (29fac2d).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #45 +/- ## ===================================== Coverage 50.3% 50.3% ===================================== Files 32 32 Lines 4386 4386 ===================================== Hits 2209 2209 Misses 2177 2177 ```

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