Fix white border on last avatar in AvatarStack (take two)

[shawnbot]

This is a fix for the white border on the last (and first, if only) avatar being visible on other backgrounds, as in our repo commit header:

I've added a story for this to Storybook to test it out:

Before	After

[jonrohan]

Looks good 👍

[pqt]

@shawnbot

There's some instances of double quotes here, whereas the rest of the file contains only single quotes. I don't think it would break anything but wanted to point it out for consistency sake.

https://github.com/primer/primer/blob/e0ef76f18416c14a29a80e76cd4f16facfe2c46b/modules/primer-avatars/stories.js#L66-L81

This line also contains a hanging space.

https://github.com/primer/primer/blob/e0ef76f18416c14a29a80e76cd4f16facfe2c46b/modules/primer-avatars/stories.js#L67