Fix a typo

This commit fixes the following issue:

```console
$ bundle exec rubocop -a
(snip)

spec/rubocop/cop/rails/redundant_allow_nil_spec.rb:92:8: C: [Correctable] InternalAffairs/ExampleDescription:
Description does not match use of expect_offense.
    it 'registers no offense' do
       ^^^^^^^^^^^^^^^^^^^^^^
```

Author: koic

spec/rubocop/cop/rails/redundant_allow_nil_spec.rb

@@ -89,7 +89,7 @@
   end
 
   context 'when using allow_nil and allow_blank true on the same helper' do
-    it 'registers no offense' do
+    it 'registers an offense' do
       expect_offense(<<~RUBY)
         validates :email, format: { with: /regexp/, allow_nil: true, allow_blank: true }
                                                     ^^^^^^^^^^^^^^^ `allow_nil` is redundant when `allow_blank` has the same value.