[css-multicol-2] Fix markup typo

Author: frivoal

css-multicol-2/Overview.bs

@@ -189,7 +189,7 @@ Spanning columns</h2>
 			the number of columns spanned will be the same as if ''column-span: all'' had been specified.
 
 			Issue: If ''column-span: 1'' does not do the same as ''column-span: none'',
-			should this behave as ''column-span: 1'' or as 'column-span: none''
+			should this behave as ''column-span: 1'' or as ''column-span: none''
 			when the element is small enough?
 	</dl>