Fix syntax typo in code example in the Context README

[tomherni]

Spotted a typo when copy/pasting a code example.

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🦋 Changeset detected

Latest commit: f47fc2d6c0b94b153e05626c6a5b65a238ebfa0c

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

This PR includes changesets to release 1 package

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@lit/context	Patch

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[tomherni]

Hi @augustjk, I think this could be merged maybe? :)