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

Thanks to @yisibl for pointing out the omission.

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;
 }