Update Simple Form templates for 3.3.0

Fixes #3146.

Please note that while internally, methods, routes etc are currently still named `sign_in`, there are still some strings in [YAML](https://github.com/plataformatec/devise/search?l=yaml&q=%22Sign+in%22&utf8=%E2%9C%93) and [HTML ERB](https://github.com/plataformatec/devise/search?l=html%2Berb&q=%22Sign+in%22&utf8=%E2%9C%93) files that use *sign in*. Is that a leftover?
Good catch! Fixed and pushed.