docs: fix typo in theming guide

[dolphingarlic]

No description provided.

[google-cla]

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[dolphingarlic]

@googlebot I signed it!

[crisbeto]

The change looks good, but the commit message doesn't match our validation. It should be something like docs: fix typo in theming guide.

[dolphingarlic]

Thanks for the heads-up @crisbeto. I've just changed the commit message

[crisbeto]

LGTM

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