docs: fix CallState.ANSWERED spelling

[thehanslevi]

Fixed #310

Summary:

• Correct CallState.ANSWRED to CallState.ANSWERED in docs/VoIP.rst.
• Align the enum heading, hangup description, and call() guidance with the source enum and existing examples.

Validation:

• rg -n "ANSWRED|ANSWERED|class CallState" docs/VoIP.rst docs/Examples.rst pyVoIP/VoIP/VoIP.py
• python3 -c "from pathlib import Path; docs=Path(\"docs/VoIP.rst\").read_text(); src=Path(\"pyVoIP/VoIP/VoIP.py\").read_text(); assert \"ANSWRED\" not in docs; assert \"ANSWERED = \\\"ANSWERED\\\"\" in src; assert \"CallState.\\\\ **ANSWERED**\" in docs; print(\"CallState docs match source enum\")"
• python3 -m py_compile pyVoIP/VoIP/VoIP.py
• git diff --check

Could not run a Sphinx docs build because this environment does not have Sphinx installed (No module named sphinx).

[TAbdiukov]

superfluous change on its own, and this is long addressed in rfcvoip

Also: use of LLMs. Quote:

Could not run a Sphinx docs build because this environment does not have Sphinx installed (No module named sphinx).