fix markup

Author: tabatkins

compositing-2/Overview.bs

@@ -290,7 +290,7 @@ The description of the 'background-blend-mode' property is as follows:
 
 <pre class='propdef'>
 Name: background-blend-mode
-Value: <'mix-blend-mode'>#
+Value: <<'mix-blend-mode'>>#
 Initial: normal
 Applies to: All HTML elements
 Inherited: no