Fix variable in data-ensure

Summary: I broke this exercise in the previous diff by making A_RADIUS dependent on itself.

Reviewers: emily, eater

Reviewed By: eater

Differential Revision: http://phabricator.khanacademy.org/D14470

exercises/multiplying_and_dividing_complex_number_polar_forms.html

@@ -171,7 +171,7 @@
                 <div data-ensure="A_RADIUS > 1 && A_RADIUS <= 10">
                     <var id="ANSWER_RADIUS">randRange(1, 10)</var>
                     <var id="B_RADIUS">randRange(1, 10)</var>
-                    <var id="A_RADIUS">A_RADIUS * B_RADIUS</var>
+                    <var id="A_RADIUS">ANSWER_RADIUS * B_RADIUS</var>
                 </div>
 
                 <var id="ANSWER_RADIUS_REP">"\\orange{" + ANSWER_RADIUS + "}"</var>