Prefer `example` over `it` for these specs.

`it "expect(...)..." doesn't read well at all. The behavior
has already been desribed by the group's doc string; each
of the individual specs is an `example` of it, so `example`
works better here.
Don’t use internally delegated matchers for generated descriptions.
Merge pull request #692 from rspec/fix-generated-descriptions-with-nested-matchers

Fix generated descriptions with nested matchers