Fix transaction example in README: transaction() is a context manager

[dhavalp5415]

The README (both .md and .rst) documents transactions as:

t = Database.transaction(); t.commit()

But Database.transaction() is decorated with @contextmanager (records.py lines 335-347): it yields a connection and handles commit/rollback internally. Following the documented example raises:

AttributeError: '_GeneratorContextManager' object has no attribute 'commit'

This PR updates both READMEs to the working usage:

with Database.transaction() as conn:
    ...

HISTORY.rst is left untouched since it records the API as it was at release time.

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Found by Doc-Drift Sentinel, an agent that verifies documentation claims against source code. The finding was verified by hand against records.py before this PR was opened.