docs: Switch typing of Action<Parameter> from null to undefined in 12-use.md

[felipesantoz]

Currently, the docs for use: give an example of using the Action interface to type an action. In it, null is specified as the optional type argument for the parameter. Unfortunately, this means the example leads to the following type error in practice, on the line where use:gestures is contained: Expected 2 arguments, but got 1.

The solution is to switch the type argument for the parameter to undefined. This gets rid of the type error, accomplishes the intended behaviour, and matches the default value for the generic to begin with.

No testing required as it is a documentation change. To recreate the type error simply copy and paste the example into any svelte project. I couldn't figure out how to get the playground compiler to do type-checking in svelte files to show it directly, or if it's even possible.

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[Rich-Harris]

preview: https://svelte-dev-git-preview-svelte-14798-svelte.vercel.app/

this is an automated message