[docs] Update List Keypairs cleos How-to

[bobgt]

Applying feedback received from EOSIO engineers during Doc-a-Thon session 2.

Change Description

Change Type

Select ONE:

• Documentation

• Stability bug fix

• Other

• Other - special case

Testing Changes

Select ANY that apply:

• New Tests

• Existing Tests

• Test Framework

• CI System

• Other

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions