docs: bootstrapping: fixed the config key
docs: bootstrapping: fixed the config key

`console.io.bootstrap: path/to/different-bootstrap.php` does not work (does not load the bootstrap file).

`bootstrap: path/to/different-bootstrap.php` does work.

phpspec version 2.2.0
ghamoron/phpspec,gnugat-forks/phpspec,Elfiggo/phpspec,danielkmariam/phpspec,shanethehat/phpspec,Harrisonbro/phpspec,nosun/phpspec,localheinz/phpspec,gnugat-forks/phpspec,javi-dev/phpspec,jon-acker/phpspec,ghamoron/phpspec,pamil/phpspec,bestform/phpspec,localheinz/phpspec,danielkmariam/phpspec,nosun/phpspec,carlosV2/phpspec,sroze/phpspec,javi-dev/phpspec,bestform/phpspec,Dragonrun1/phpspec,sroze/phpspec,jon-acker/phpspec,shanethehat/phpspec,Elfiggo/phpspec,docteurklein/phpspec,Dragonrun1/phpspec,pamil/phpspec,docteurklein/phpspec,carlosV2/phpspec,Harrisonbro/phpspec