fix(docs): tanstack form docs

[Psycarlo]

📚 Description

TanStack Form Demo binds
field.handleChange to @input.

When typing, it will make the input to display [object InputEvent] instead of the text typed by the user because field.handleChange is getting the event instead of event.target.value.

🔄 Changes

From

@input="field.handleChange"

To

@input="field.handleChange($event.target.value)"