Fix incorrect path to bootstrap.scss

The command to copy the bootstrap.scss file (the one with all components listed) specifies the incorrect path.

It says 

cp $(bundle show bootstrap-sass)/vendor/assets/stylesheets/bootstrap.scss \
app/assets/stylesheets/bootstrap-custom.scss

when it should say 

cp $(bundle show bootstrap-sass)/vendor/assets/stylesheets/ BOOTSTRAP /bootstrap.scss ...