Add 'flex' attribute to element in the 'layout attributes' page

[lucasloisp]

No description provided.

[ebidel]

Why the change?

[arthurevans]

I had noticed this -- the text is incorrect here. It says, " to make "Gamma" 2x larger than "Beta" and "Alpha" 3x larger, use:"

But if you look at the example, Beta is less than 1/2 the size of Gamma, because it's size auto.

We could correct the description instead, but correcting the sample is easier.

[addyosmani]

@lucasloisp thanks for the PR. For future ones, could you include a description of why the change is needed? It helps us understand your intentions :)

[lucasloisp]

Sure. Sorry for the misunderstanding. That was my point. That Gamma wasn't 2x larger than Beta, it was actually smaller. I thought of changing the description, but believed it would better suit the purpose to make them all flex, plus following the previous example, where "Beta" does flex.
I'm still new to the world of open-source code, so thanks for the response anyways

[ebidel]

Before this PR can be merged, please sign the CLA. See the CONTRIBUTING guide.

[lucasloisp]

done
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[ebidel]

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