Suppress RuboCop offense

This commit suppresses the following new RuboCop offense with RuboCop RSpec 3.0.2:

```console
$ bundle exec rake
(snip)

spec/project_spec.rb:27:9: C: [Correctable] RSpec/PredicateMatcher: Prefer using nil? over be_nil matcher.
        expect(start_with_subject).to( ...
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

294 files inspected, 1 offense detected, 1 offense autocorrectable
```

spec/project_spec.rb

@@ -24,8 +24,8 @@
         start_with_subject = description.match(/\AThis cop (?<verb>.+?) .*/)
         suggestion = start_with_subject[:verb]&.capitalize if start_with_subject
         suggestion ||= 'a verb'
-        expect(start_with_subject).to(
-          be_nil, "`Description` for `#{name}` should be started with `#{suggestion}` instead of `This cop ...`."
+        expect(start_with_subject.nil?).to(
+          be(true), "`Description` for `#{name}` should be started with `#{suggestion}` instead of `This cop ...`."
         )
       end
     end