Fix typo in a Precognition for Alpine.js example

[johnRivs]

In the Precognition for Alpine.js example where it's shown how to manually submit the form, form is being referenced instead of this.form.

This PR will result in:

<form @submit.prevent="submit" x-data="{
    form: /* ... */,
    submit() {
        this.form.submit()
            .then(() => {
                // form.reset();
                this.form.reset();
            });
    },
}">