diff --git a/book/index.html b/book/index.html
index 949fe076..76efb0a2 100644
--- a/book/index.html
+++ b/book/index.html
@@ -1726,7 +1726,7 @@ <h2 id="observerpatternjavascript">
 
 <ul>
   <li><strong>Subject</strong>: maintains a list of observers, facilitates adding or removing observers</li>
-  <li><strong>Observer</strong>: provides a update interface for objects that need to be notified of a Subject's changes of state</li>
+  <li><strong>Observer</strong>: provides an update interface for objects that need to be notified of a Subject's changes of state</li>
   <li><strong>ConcreteSubject</strong>: broadcasts notifications to observers on changes of state, stores the state of ConcreteObservers</li>
   <li><strong>ConcreteObserver</strong>: stores a reference to the ConcreteSubject, implements an update interface for the Observer to ensure state is consistent with the Subject's</li>
 </ul>