`opentelemetry-otlp` documentation mentioning `tls-tonic` feature flag that does not exist

[lochetti]

At the feature flags section of the documentation, the feature flag tls-tonic is mentioned, but when we look at the available feature flags it does not exist at version 0.26.0.

So maybe it is just a matter of removing the bullet point that mentions tls-tonic from the documentation?

[lalitb]

@lochetti Thanks for raising the issue. If you would like to contribute the fix?