Fix spelling

Summary: Noticed a couple of typos in the docs, and then things got out of hand.

Test Plan:
  - Stared at the words until my eyes watered and the letters began to swim on the screen.
  - Consulted a dictionary.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, yelirekim, PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D18693

src/infrastructure/diff/view/PHUIDiffGraphView.php

@@ -160,7 +160,7 @@ final class PHUIDiffGraphView extends Phobject {
 
             $terminated[$ii] = true;
 
-            // If this thread is joinining some other node here, we don't want
+            // If this thread is joining some other node here, we don't want
             // to terminate it.
             if (isset($graph[$key + 1])) {
               $joins = $graph[$key + 1]['join'];