Fixed typo in demo description

Author: jzaefferer

demos/button/splitbutton.html

@@ -49,7 +49,7 @@
 
 <div class="demo-description">
 
-<p>An example of a split button built with two buttons: A plan button with just text, one with only a primary icon
+<p>An example of a split button built with two buttons: A plain button with just text, one with only a primary icon
 and no text. Both are grouped together in a set.</p>
 
 </div><!-- End demo-description -->