Update rspec-rails documentation links in README

[josienb]

There are two rspec-rails documentation links in the README to the old relishapp documentation. I've replaced them with updated links to the new docs:

• https://relishapp.com/rspec/rspec-rails/v/4-0/docs/directory-structure to https://rspec.info/features/6-0/rspec-rails/directory-structure/
• https://relishapp.com/rspec/rspec-rails/docs/system-specs/system-spec#system-specs-driven-by-selenium-chrome-headless to https://rspec.info/features/6-0/rspec-rails/system-specs/system-specs

This also changes the doc links from v4 to v6 - not sure if that's acceptable?