[css-overflow-3] Fixup bibliography

Author: frivoal

css-overflow-3/Overview.bs

@@ -66,7 +66,7 @@ Introduction</h2>
 	which would likely cause
 	that content to overlap with other elements.
 
-	CSS Level 2 [[CSS21]] introduced the 'overflow' property,
+	CSS Level 2 [[CSS2]] introduced the 'overflow' property,
 	which allows authors to have overflow be handled by scrolling,
 	which means it is no longer an authoring error.
 	It also allows authors to specify
@@ -330,7 +330,7 @@ Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow'
 		Name: overflow-x, overflow-y
 		Value: visible | hidden | clip | scroll | auto
 		Initial: ''visible''
-		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
+		Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		Inherited: no
 		Percentages: N/A
 		Computed value: as specified, except with ''visible''/''clip'' computing to ''overflow/auto''/''hidden'' (respectively) if one of 'overflow-x' or 'overflow-y' is neither ''visible'' nor ''clip''
@@ -348,7 +348,7 @@ Scrolling and Clipping Overflow: the 'overflow-x', 'overflow-y', and 'overflow'
 		Name: overflow
 		Value: [ visible | hidden | clip | scroll | auto ]{1,2}
 		Initial: visible
-		Applies to: block containers [[!CSS21]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
+		Applies to: block containers [[!CSS2]], flex containers [[!CSS3-FLEXBOX]], and grid containers [[!CSS3-GRID-LAYOUT]]
 		Inherited: no
 		Percentages: N/A
 		Computed value: see individual properties