Update source/tutorial/deploy-replica-set.txt

Fixed the requirements section to agree with the docs here:

http://docs.mongodb.org/manual/core/replication/

The requirement for a set is two or more, but the tutorial describes
three, and it led to some confusion in the community:

http://serverfault.com/questions/462780/does-a-mongodb-replica-set-require-at-least-2-or-3-members