Minor typo in the documentation for v1.6.8

[HeinrichWarkentin]

In:
https://pyvoip.readthedocs.io/en/v1.6.8/VoIP.html#callstate

VoIP.CallState
CallState is an Enum with four attributes.

CallState.DIALING
This CallState is used to describe when a user has originated a call to a client, but it has yet to be answered.

CallState.RINGING
This CallState is used to describe when a client is calling, but the call has yet to be answered.

In this state, you can use VoIPCall.answer() or VoIPCall.deny().

CallState.ANSWRED <- ANSWERED
This CallState is used to describe when a call has been answered and is active.

In this state, you can use VoIPCall.hangup().

CallState.ENDED
This CallState is used to describe when a call has been terminated.

In this state, you can not use any functions.