Modals not centered on mobile (viewport width <640px)
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**What component (if applicable)**
- URL for category: https://tailwindui.com/components/application-ui/overlays/modals
- Component name: Modals
**Describe the bug**
Modals are not centered on mobile (<640px); instead they are a fixed padding away from the bottom of the viewport.
**To Reproduce**
Steps to reproduce the behavior:
1. Go to https://tailwindui.com/components/application-ui/overlays/modals;
2. In any example, drag the viewport divider all the way to the left;
3. The modal stops being centered once the viewport is narrower than 640px; instead, the modal is always 5rem from the bottom of the viewport due to a `pb-20`.
**Expected behavior**
Modals are always centered, including on mobile.
**Screenshots**
<img src="https://user-images.githubusercontent.com/4149852/108954689-2575ea00-76a8-11eb-92db-6cf1f60852fa.png" width="300">
**Browser/Device (if applicable)**
N/A, not OS/browser dependent.