Update DataGrid.xml (dotnet#9034)

Author: miloush

xml/System.Windows.Controls/DataGrid.xml

@@ -808,7 +808,7 @@
           <format type="text/markdown"><![CDATA[
 
 ## Remarks
- By default, the user can delete rows by selecting one or more rows and pressing the DELETE key. Deleting a row removes the item that the row represents from the <xref:System.Windows.Controls.ItemsControl.ItemsSource%2A>. If the <xref:System.Windows.Controls.ItemsControl.ItemsSource%2A> does not allow deletions, the user cannot delete rows even if this property is set to `true`.
+ By default, the user can delete rows by selecting one or more rows and pressing the <kbd>Delete</kbd> key. Deleting a row removes the item that the row represents from the <xref:System.Windows.Controls.ItemsControl.ItemsSource%2A>. If the <xref:System.Windows.Controls.ItemsControl.ItemsSource%2A> does not allow deletions or if the <xref:System.Windows.Controls.DataGrid> is read-only, the user cannot delete rows even if this property is set to `true`.
 
  ]]></format>
         </remarks>