Fix mistake in flex-justify-start explanation

[dcastil]

When using

.container {
  display: flex;
  justify-content: flex-start;
  flex-direction: column;
}

the start line is at the top of the container, not the end line.

Source: https://developer.mozilla.org/en-US/docs/Web/CSS/justify-content

/cc @primer/ds-core

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[dcastil]

Please let me know if anything is missing in this PR. I‘m on a mobile phone on a slow internet connection at the moment and wasn‘t able to read any issues or READMEs.

[simurai]

Please let me know if anything is missing in this PR.

@dcastil All good, thanks for fixing the docs. 🙇 ❤️ It will go in our next release.