Add missing comma

Author: ugultopu

css-cascade-4/Overview.bs

@@ -415,7 +415,7 @@ Resetting All Properties: the 'all' property</h3>
 	[[css-writing-modes-3]]
 
 	<div class='example'>
-		For example, if an author specifies ''all: initial'' on an element
+		For example, if an author specifies ''all: initial'' on an element,
 		it will block all inheritance and reset all properties,
 		as if no rules appeared in the author, user, or user-agent levels of the cascade.