Fix a typo

Summary: Self-explanatory.

Test Plan: Read it.

Reviewers: avivey, epriestley, #blessed_reviewers

Reviewed By: avivey, epriestley, #blessed_reviewers

Subscribers: avivey, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D13063

Author: joshuaspence

README.md

@@ -44,7 +44,7 @@ A handful of solid utility functions.
 
 `libphutil` is used by
  - [Phabricator](https://secure.phabricator.com/diffusion/P/)
- - [Arcnaist](https://secure.phabricator.com/diffusion/ARC/)
+ - [Arcanist](https://secure.phabricator.com/diffusion/ARC/)
  - [Diviner](https://secure.phabricator.com/book/phabricator/article/diviner/)