[system][docs] Update typo on the sx prop page

[bricker]

• [x] I have followed (at least) the PR section of the contributing guide.

[mui-bot]

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• docs/data/system/getting-started/the-sx-prop/the-sx-prop.md

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[danilo-leal]

Heya @bricker — appreciate the PR! Can I ask you to open it again but pointing to next as the base branch instead of master?

[bricker]

@danilo-leal whoops... done!