[docs] Post release instructions - broken link

[marcalff]

In RELEASING.md, the following section has a broken link:

## Post Release

Update the OpenTelemetry.io document
[here](https://github.com/open-telemetry/opentelemetry.io/tree/main/content/en/docs/cpp)
by sending a Pull Request.

This was caused by
Move languages under /docs/instrumentation #963

Fix: docs/cpp -> docs/instrumentation/cpp