fix example so that attachShadow does not throw an exception

[dylanb]

An unordered list <ul> is one of the elements that does not allow attachShadow() to be called. The example tries to call attachShadow() on the <ul>. This commit fixes this problem.

[hayatoito]

Thanks. However, it looks that this example needs more fix because the shadow host's children are <li>. I am afraid that it does not make sense to use <li> here if we do not use <ul> as a shadow host..

And the HTML Standard does not guarantee that <ul><slot></slot></ul> work at all. Even if <li> is assigned to a slot, we can not say that <ul> and <li> are in parent-child relationship here. This issue is still in the air.