Fix json_stream copy error typo

[thehanslevi]

Summary

• correct the copy/deepcopy error message from json_steam to json_stream
• update the existing not-copyable test expectations to match

Tests

• git diff --check
• python3 -m py_compile src/json_stream/base.py src/json_stream/tests/test_loader.py
• python -m unittest src/json_stream/tests/test_loader.py (21 tests)
• python -m unittest discover -s src (80 tests; 1 expected failure)
• python -m flake8 . --count --select=E9,F63,F7,F82 --show-source --statistics
• python -m flake8 . --count --exit-zero --max-complexity=10 --max-line-length=120 --statistics
• python -m pytest --cov=src --cov-report=term with rust tokenizer (79 passed, 1 xfailed)
• python -m pytest --cov=src --cov-report=term with the repo's python tokenizer stub (72 passed, 7 skipped, 1 xfailed)