Fix spelling

Author: jasondavies

book/src/en-US/performance.xml

@@ -127,7 +127,7 @@ $(document).ready(PI.onReady);            </programlisting>
     <title>Optimize Selectors</title>
 
     <para>Selector optimization is less important than it used to be, as more
-    browser implement <code>document.querySelectorAll()</code> and the burden
+    browsers implement <code>document.querySelectorAll()</code> and the burden
     of selection shifts from jQuery to the browser. However, there are still
     some tips to keep in midn.</para>
 

book/src/en-US/plugins.xml

@@ -54,7 +54,7 @@
 
     <para>This has the effect of creating a "private" scope that allows us to
     extend jQuery using the dollar symbol without having to risk the
-    possibility that the dollar has been over-written by another
+    possibility that the dollar has been overwritten by another
     library.</para>
 
     <para>So our actual plugin, thus far, is this:</para>