Docs: update installation for bson gem 4.0
Docs: update installation for bson gem 4.0

ukblewis/bson-ruby,estolfo/bson-ruby,estolfo/bson-ruby,mongodb/bson-ruby,ukblewis/bson-ruby,ukblewis/bson-ruby,estolfo/bson-ruby,mongodb/bson-ruby,estolfo/bson-ruby,mongodb/bson-ruby,ukblewis/bson-ruby,mongodb/bson-ruby