Fixes a typo in the docs.
Fixes a typo in the docs.

mongodb/bson-ruby,agis-/bson-ruby,ukblewis/bson-ruby,johnsimon/bson-ruby,ukblewis/bson-ruby,estolfo/bson-ruby,mongodb/bson-ruby,johnsimon/bson-ruby,estolfo/bson-ruby,mongodb/bson-ruby,ukblewis/bson-ruby,hudon/bson-ruby,agis-/bson-ruby,mongodb/bson-ruby,ukblewis/bson-ruby,estolfo/bson-ruby,estolfo/bson-ruby,hudon/bson-ruby,agis-/bson-ruby,johnsimon/bson-ruby,hudon/bson-ruby