Fix typo / duplication in developer docs.
Fix typo / duplication in developer docs.

jskeet/nodatime,zaccharles/nodatime,malcolmr/nodatime,zaccharles/nodatime,nodatime/nodatime,BenJenkinson/nodatime,malcolmr/nodatime,zaccharles/nodatime,malcolmr/nodatime,zaccharles/nodatime,nodatime/nodatime,zaccharles/nodatime,zaccharles/nodatime,malcolmr/nodatime,BenJenkinson/nodatime,jskeet/nodatime