3to4: Fix suggested replacements for border-width

The new CSS border-spacing does what Grid::(row|column)_spacing and
Box::spacing already did, i.e. controlling the space added between child
widgets, so it’s not a replacement for Container::border-width.

docs/reference/gtk/migrating-3to4.xml

@@ -169,7 +169,7 @@
       <para>
         GTK+ 4 has removed the #GtkContainer::border-width property.
         Use other means to influence the spacing of your containers,
-        such as the CSS border-spacing property.
+        such ss the CSS margin and padding properties on child widgets.
       </para>
     </section>