fix(VoiceReceiver): delete opus encoder from map in stoppedSpeaking

[SpaceEEC]

Please describe the changes this PR makes and why it should be merged:

This PR fixes VoiceReceivers failing to decode PCM data after the first silence of a user.

The cause is that the relevant OpusEncoder got reused after being destroyed.
It was destroyed here, but not removed from the map like it should.

discord.js/src/client/voice/receiver/VoiceReceiver.js

Lines 113 to 115 in 505df2e

	if (opusEncoder) {
	opusEncoder.destroy();
	}

After removing it from the map a new one was created in case the user starts speaking again and valid PCM data could be decoded.

This PR also fixes the documentation about the PCM format, it's actually s16 and not s32.

Status

• Code changes have been tested against the Discord API, or there are no code changes
• I know how to update typings and have done so, or typings don't need updating

Semantic versioning classification:

• This PR changes the library's interface (methods or parameters added)
  • This PR includes breaking changes (methods removed or renamed, parameters moved or removed)
• This PR only includes non-code changes, like changes to documentation, README, etc.