Add an example of nep17 balance NeoGo CLI usage

[AnnaShaleva]

Preparation part contains the step that aimed to check the GAS balance, and it's done via RPC call:

[Preparation.](https://github.com/nspcc-dev/neo-go-sc-wrkshp#workshop-preparation) Learn how to run a Neo private network, transfer some funds from multi-signature account to a simple signature account using NeoGo CLI and check the balance via JSON RPC call.

We have a separate workshop part about RPC calls and doesn't have any nep17 CLI command mention. What we need is to replace this step with CLI balance checking. For better user experience.