Fix small mistake in README.md

[francois-ferrandis]

As far as I know:

• to prepend is to add at the beginning
• to append is to add at the end

Simple Form adds an asterisk at the end of the label for required fields.

[nashby]

Prepend is what SimpleForm does by default, see https://github.com/heartcombo/simple_form/blob/c2c7faf3e532072e1ef511a97d677ae0e7c38957/lib/simple_form.rb#L104

but you can configure it any way you like (that's probably what you have in your app)

[francois-ferrandis]

Woops, sorry, I should have been more thorough. :grimacing:

Thank you for your time! :pray: