docs(guide/expression): fix duplicate key error in example

The example that demonstrates how to parse expressions can fail if you
pass in the same expression twice.  By using "track by $index" we can
fix this.

Closes #4472

docs/content/guide/expression.ngdoc

@@ -70,7 +70,7 @@ You can try evaluating different expressions here:
    <input type='text' ng-model="expr" size="80"/>
    <button ng-click="addExp(expr)">Evaluate</button>
    <ul>
-    <li ng-repeat="expr in exprs">
+    <li ng-repeat="expr in exprs track by $index">
       [ <a href="" ng-click="removeExp($index)">X</a> ]
       <tt>{{expr}}</tt> => <span ng-bind="$parent.$eval(expr)"></span>
      </li>