Update DrawingContext.xml (#3729)

Fixed minor spelling error

Author: Cruiser18

xml/System.Windows.Media/DrawingContext.xml

@@ -453,7 +453,7 @@
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
- The brush and the pen can both be `null`.  If the brush is `null`, then no fill is performed.  If the pen is `null`, then no stoke is performed.  If both the pen and the brush are `null`, then the drawing is not visible.  
+ The brush and the pen can both be `null`.  If the brush is `null`, then no fill is performed.  If the pen is `null`, then no stroke is performed.  If both the pen and the brush are `null`, then the drawing is not visible.  
   
  ]]></format>
         </remarks>