Mention DeleteObjectReference when talking about Orleans Observers

[Tragetaschen]

https://github.com/dotnet/docs/blob/main/docs/orleans/grains/observers.md

The observers page teaches calling CreateObjectReference to enable an Orleans client to register an IGrainObserver with the library. It does not mention that this call needs to be paired with DeleteObjectReference.

If the latter is not called when done—at least for the OutsideRuntimeClient—a small reference remains in the InvokableObjectManager.localObjects dictionary, effectively becoming a memory leak, that will kill the (client) process eventually.