fix: correct build-reth.sh default repo and version in README

[consumeobeydie]

What changed? Why?

The build-reth.sh description in clients/README.md contained three incorrect values in the Default Configuration section that contradicted both the actual script and the Available Environment Variables section of the same README.

Before:

• Description: "Builds the reth binary from the Paradigm reth repository using Cargo."
• Repository: https://github.com/paradigmxyz/reth/
• Version: main

After:

• Description: "Builds the op-reth binary from the Ethereum Optimism monorepo using Cargo."
• Repository: https://github.com/ethereum-optimism/optimism/
• Version: develop

The script (build-reth.sh) actually clones ethereum-optimism/optimism and builds op-reth using the develop branch as the default. The Available Environment Variables section (lines 97–101) already documented this correctly — this PR fixes the inconsistent Default Configuration section to match.

Notes to reviewers

No code changes — documentation only. Verified against clients/build-reth.sh lines 10–11.

How has it been tested?

Cross-referenced clients/build-reth.sh defaults with the corrected README values.

[cb-heimdall]

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