Update CornerRadius.xml (#4736)

Author: friedcucumber

xml/System.Windows/CornerRadius.xml

@@ -26,14 +26,14 @@
     </Attribute>
   </Attributes>
   <Docs>
-    <summary>Represents the radii of a rectangle's corners.</summary>
+    <summary>Represents the radius of a rectangle's corners.</summary>
     <remarks>
       <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
  This class has four properties, <xref:System.Windows.CornerRadius.TopLeft%2A>, <xref:System.Windows.CornerRadius.TopRight%2A>, <xref:System.Windows.CornerRadius.BottomLeft%2A>, and <xref:System.Windows.CornerRadius.BottomRight%2A>, corresponding to the individual corners of the rectangle.  Each is a <xref:System.Double> structure that can be set independently of the others.  
   
- The radii cannot be negative.  
+ The radius cannot be negative.  
   
 <a name="xamlAttributeUsage_Duration"></a>   
 ## XAML Attribute Usage