fix: balance parentheses in error about wrong content type for action

[danieldiekmeier]

When you send the wrong content type to an action, SvelteKit throws a helpful error, but it's missing the parentheses at the end:

Actions expect form-encoded data (received application/json;
                                                            ^ Whoops

This PR adds it. That's all! Thanks!

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[benmccann]

Another alternative would be to remove the parentheses. I'll leave this as a suggestion to see if the other maintainers have any preference

Suggested change

	`Actions expect form-encoded data (received ${event.request.headers.get('content-type')})`
	`Actions expect form-encoded data, but received ${event.request.headers.get('content-type')}`