[Popover] Fix inconsistent border-radius

[beefchimi]

A quick one two-liner.

I noticed that the border-radius for <Popover /> was funky, and tracked it down to inconsistent border-radius values.

This PR resolves the visual issue.

Before

Here you can see some of the Popover background poking through in the top left and right corner:

After

Top corners now resolved:

[ghost]

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[danrosenthal]

@HYPD or @sarahill what is the intended border radius on this component? Should it be large or just the base?

[HYPD]

@danrosenthal base would be the correct value to use here.

@sarahill @ry5n Should we document these values in the style guide and ui kit? If base and large are available in code, designers may be asked which one to use. We do this with spacing already here
https://polaris.shopify.com/design/spacing#section-spacing-in-code

[beefchimi]

@HYPD updated to use the base variant

[ghost]

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