[COMP] Flex: fix responsive classnames

[andrewrubin]

Description

Adds a missing dash to the breakpoint classnames. Fixes #172

Additional Notes

As we discussed in #172, we ought to update the buildBreakpointClassnames utility to not only honor the large-up breakpoints, as that is a specific grid-layout opinion. The <Flex> component, for example, should be able to receive specific values for small and medium as well. I am making a separate issue for that though!

Screenshots

This screencap illustrates the justify prop using the following responsive options:

<Flex justify={{ md: "flex-start", lg: "center", xxl: "flex-end" }}>
  <span>I</span>
  <span>am</span>
  <span>so</span>
  <span>cool</span>
</Flex>

https://github.com/wethegit/component-library/assets/30575213/63376902-da97-4607-a698-5d8c186cbcd2

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