Typos in gRPC instrumentation example codes

[ymotongpoo]

Describe your environment

• Platform: any
• Python version: any
• Library version: main HEAD as of Aug 24th

Steps to reproduce
See documents for gRPC instrumentation

What is the expected behavior?
For client instrumentation example, I would see either of the followings:

1. GrpcInstrumentorClient().instrument() is called without return value assignment.
2. Get the return value of GrpcInstrumentorClient() and call instrument() method of it.

What is the actual behavior?

I see the following example codes.

In the Usage Client section:

instrumentor = GrpcInstrumentorClient().instrument()

And in the GrpcInstrumentorClient class document:

grpc_client_instrumentor = GrpcInstrumentorClient()
grpc.client_instrumentor.instrument()

where 2nd line is calling non existing package. It should be typo of the variable defined in 1st line.

Additional context

I was trying implement the code to fix #373 and found this issue, so I created separate issue here. I'll create a PR for this shortly.