Update a documentation about #be_an_alias_of matcher

[DenisZackharov]

I think that documentation need to tell about connecting a #be_an_alias_of matcher with adding a require "action_policy/rspec", like you wrote it for #be_authorized_to matcher

#be_an_alias_of screen:

#be_authorized_to screen:

[DenisZackharov]

I made a PR #243

[palkan]

Closed by #243