fix(README): synchronous use of event in effect

[julien-f]

The event must be used directly in the effect, not in the async state updater.

I could have used directly param destructuring to get the value but I thought it would complexify things unnecessarily.

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- `2018-01-04T14:42:43.093Z:abbc1fa`: The pull request was created - `2018-07-06T08:58:46.215Z:abbc1fa`: @divmain selected `documentation only` as the semantic version.