Added \cong + fixed comment copypasta.

Test Plan: Parses and renders locally.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D6393

buildTree.js

@@ -423,6 +423,7 @@ var charLookup = {
     "\\cdot": "\u22c5",
     "\\circ": "\u2218",
     "\\colon": ":",
+    "\\cong": "\u2245",
     "\\div": "\u00f7",
     "\\ge": "\u2265",
     "\\geq": "\u2265",