Update skeleton.css

[zignis]

Solves the unwanted misplacement of skeleton child (used to have a gap at the top in rare cases).

[srmagura]

Hey @hexm7, would you be able to provide a CodeSandbox that shows the problem this PR solves?

[zignis]

Here is the demo: https://codesandbox.io/s/serverless-darkness-2ipti?file=/src/App.js
Everything is fine on the initial render, but as you change something in the CSS file you'll notice the animated element getting shifted a bit. I have attached a screenshot.

[srmagura]

I'm just seeing the default React template when I visit that link. Did you send the correct link?

[zignis]

I'm extremely sorry as I'm new to CodeSandBox. Here's the StackBlitz project: https://stackblitz.com/edit/react-vqvafn?file=src%2FApp.js

[srmagura]

Thanks @hexm7! I don't understand why top: 0 is required in this case, but it shouldn't have any negative side effects. I will work on getting this ready for release.