ui-icon-button docs have typo

[Adam Jagosz]

The docs say:

You can set a custom or SVG icon using the icon slot.

but in the Slots tab there is no icon slot listed, the above should be:

You can set a custom or SVG icon using the default slot.

[Josephus Paye II]

Good catch 😄. PR welcome.

[Josephus Paye II]

Fixed in #457.