[css-values-5] Add overflow:hidden to example in calc-size() explainer.

Thanks to @yisibl for pointing out the omission.

Author: dbaron

css-values-5/calc-size-explainer.md

@@ -143,6 +143,7 @@ details::details-content {
   --open-close-duration: 500ms;
   display: block; /* override default 'display: contents' */
   height: 0;
+  overflow: hidden;
   transition: height var(--open-close-duration),
               content-visibility var(--open-close-duration) allow-discrete step-end;
 }