docs: 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 90.49%. Comparing base (d7def91) to head (2aa2254).

Additional details and impacted files

@@               Coverage Diff                @@
##           forks/amsterdam    #2936   +/-   ##
================================================
  Coverage            90.49%   90.49%           
================================================
  Files                  535      535           
  Lines                32430    32430           
  Branches              3012     3012           
================================================
  Hits                 29349    29349           
  Misses                2563     2563           
  Partials               518      518           

Flag	Coverage Δ
unittests	90.49% <ø> (ø)

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