Fix Typographical Errors in Documentation Files

[Dimitrolito]

Problem

Several typographical errors were found and corrected in the documentation files.

Summary of Changes

• Fixed hyphenation issues with time-related terms (e.g., "2-second" and "5-minute") in the metrics.md file.
• Corrected redundancy in the use of "much much" to "much" in the leader-rotation.md file.
• Fixed the incorrect use of the article "an" instead of "a" in the rpc-transaction-history.md file.

[anza-team]

Thank you for the contribution! This repository is no longer in use. Please re-open this pull request in the agave repo: https://github.com/anza-xyz/agave