readme: fix typo in example code comment
Module: introduce x-module() function

x-module() function returns the placeholder selector for a module and
it's just '%<module-name>' selector for now.

Now, x-extend() for the selectable extending requires
set(s) of selector prefix and suffix(es).

    For example:
    @include x-extend( (
        '%module': (
            '-modifier'
        )
    ) );

or

    @include x-extend( (
        x-module('module'): (
            x-modifier('modifier')
        )
    ) );