Use RSpec module on describe calls in docs

Created by: peagha

Hello! I've noticed that the README in https://relishapp.com/rspec/rspec-core/v/3-8/docs didn't use the RSpec module on the describe call; the module is used in mostly all examples across the documentation: 1(https://relishapp.com/rspec/rspec-core/v/3-8/docs/subject/implicitly-defined-subject), 2(https://relishapp.com/rspec/rspec-core/v/3-8/docs/subject/explicit-subject), 3(https://relishapp.com/rspec/rspec-core/v/3-8/docs/formatters/configurable-colors).

I've took the opportunity to also update other places that didn't use the RSpec module.

To find those I used this regex, just in case is useful to someone: \sdescribe [A-Z] (any describe prefixed with a space where the subject starts with a capital letter, presumably a class)