docs(spec-specs): fix TransactionEnvironment docstring

[nanookclaw]

Summary

Fixes the TransactionEnvironment docstring in the Berlin VM module so it describes transaction-level processing instead of repeating the Message wording about contract creation or message calls.

The issue points to this as a copy-paste error from Message; this PR keeps the change scoped to that documented Berlin fork location and leaves behavior untouched.

Closes #2929

Verification

• uv run ruff format --check src/ethereum/forks/berlin/vm/__init__.py
• uv run ruff check src/ethereum/forks/berlin/vm/__init__.py
• git diff --check

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.50%. Comparing base (cb77cc5) to head (8cef715).

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #2936   +/-   ##
================================================
  Coverage            90.50%   90.50%           
================================================
  Files                  535      535           
  Lines                32407    32407           
  Branches              3011     3011           
================================================
  Hits                 29331    29331           
  Misses                2559     2559           
  Partials               517      517           

Flag	Coverage Δ
unittests	90.50% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.