update default for flex wrap styles

[arelia]

What are you trying to accomplish?

• updates documentation to indicate that the default wrap style for flexbox is flex-wrap: no-wrap

What approach did you choose and why?

• Per the W3 spec no-wrap is the default value
• I haven't found the declaration for .d-flex in https://github.com/primer/css, but looking at how the CSS is applied in the browser it doesn't look like Primer sets wrap on that class either, so it wouldn't be the default for an application using Primer (although our application may be behind in Primer versions)

What should reviewers focus on?

• Is this change correct? Have I missed something in Primer or misunderstood the spec?
• If this change is correct should the flex-nowrap line be first in the list in the documentation, as the default?

Can these changes ship as is?

• Yes, this PR does not depend on additional changes. 🚢

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Latest commit: b99ce07

The changes in this PR will be included in the next version bump.

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@primer/css	Patch

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[simurai]

Is this change correct? Have I missed something in Primer or misunderstood the spec?

Yes, this looks correct. Thanks. 🙇

If this change is correct should the flex-nowrap line be first in the list in the documentation, as the default?

Usually the default is written first, so yeah, I would say moving .flex-nowrap to the top would be great. 👍