fix typo

Author: Marcos Cáceres

index.html

@@ -170,7 +170,7 @@ <h4>
             return <a>a promise rejected with</a> with a "{{NotAllowedError}}"
             {{DOMException}}.
             </li>
-            <li>Set <a>[[\sharePromise]]</a> be <a>a new promise</a>.
+            <li>Set <a>[[\sharePromise]]</a> to be <a>a new promise</a>.
             </li>
             <li>Return <a>[[\sharePromise]]</a> and <a>in parallel</a>:
               <ol>