doc: small typo in domain.markdown
doc: small typo in domain.markdown

per: https://github.com/joyent/node/pull/7715

originally submitted by @tonylukasavage

Reviewed By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/2378

dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node,dreamllq/node