Add example for rule vue/no-restricted-static-attribute

[youxingz]

From the conversation of #2020, after some unnecessary work and the valuable comments from @FloEdelmann, I then added a example to the document to make it clearer.

The example of the element property about the rule vue/no-restricted-static-attribute:

options = [
    {
        "key": "style",
        "element": /^([A-Z$][a-zA-Z_$]*)$/,
        "message": "Using \"style\" is not allowed in custom component. Use \"class\" instead."
    }
]