docs: fix grammar in docs/running_tests/test_formats (batch 2)

[danceratopz]

Fix grammar issues in docs/running_tests/test_formats:

• blockchain_test.md: "post merge" -> "post-merge"
• blockchain_test.md: fix blobVersionedHashes description (was copy of maxFeePerBlobGas)
• blockchain_test_engine.md: "a engine" -> "an engine"
• blockchain_test_engine_x.md: "If post field" -> "If the post field"
• transaction_test.md: "test client's" -> "test a client's"

🗒️ Description

🔗 Related Issues or PRs

N/A.

✅ Checklist

• All: Ran fast tox checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  uvx tox -e static

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered adding an entry to CHANGELOG.md.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).
• Tests: Ran mkdocs serve locally and verified the auto-generated docs for new tests in the Test Case Reference are correctly formatted.
• Tests: For PRs implementing a missed test case, update the post-mortem document to add an entry the list.
• Ported Tests: All converted JSON/YML tests from ethereum/tests or tests/static have been assigned @ported_from marker.

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