Playback example module contains replace directive

[galsjel]

https://github.com/pion/opus/blob/70baa277ae2f915149372e5ca45d4dd2460a1239/examples/playback/README.md?plain=1#L9

Running this command produces an error message:

go: github.com/pion/opus/examples/playback@latest (in github.com/pion/opus/examples/[email protected]):
	The go.mod file for the module providing named packages contains one or
	more replace directives. It must not contain directives that would cause
	it to be interpreted differently than if it were the main module.