[css-backgrounds-3] [css-shapes-1] Update refs to the box model (#3078)

Because we have a new css-box-3 spec now.

Author: xfq

css-backgrounds-3/Overview.bs

@@ -39,7 +39,7 @@ spec:css-values-3; type:type; text:<position>
 <p><em>This subsection is not normative.</em>
 
 <p>When elements are rendered according to the
-<a href="https://www.w3.org/TR/CSS2/box.html">CSS box model</a> [[!CSS2]],
+<a href="https://www.w3.org/TR/css-box-3/#the-css-box-model">CSS box model</a> [[!CSS-BOX-3]],
 each element is either not displayed at all, or formatted as one
 or more rectangular boxes. Each box has a rectangular content
 area, a band of padding around the content, a border around the
@@ -51,7 +51,7 @@ negative, but margins have no influence on the background and border.)
   content, padding, border and margin areas">
 
   <p class="caption">The various areas and edges of a typical box.
-  (This diagram is explained in the CSS2.1 Box Model chapter [[!CSS2]].)
+  (This diagram is explained in the CSS Box Model Module [[!CSS-BOX-3]].)
 </div>
 
 <p>The properties of this module deal with the decoration of the border

css-shapes-1/Overview.bs

@@ -706,7 +706,7 @@ Shapes from Box Values</h2>
 
 	Shapes can be defined
 	by reference to edges in the
-	<a href="https://www.w3.org/TR/CSS21/box.html#box-dimensions">CSS Box Model</a>.
+	<a href="https://www.w3.org/TR/css-box-3/#the-css-box-model">CSS Box Model</a>.
 	These edges include
 	<a href="https://www.w3.org/TR/css3-background/#corner-shaping">border-radius curvature</a> [[!CSS3BG]]
 	from the used border-radius values.