Fix a build error

This commit fixes the following build error.

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

Offenses:

spec/rubocop/cop/rails/where_missing_spec.rb:258:5: C: [Correctable]
Layout/RedundantLineBreak: Redundant line break detected.
    it 'does not register an offense when ' \ ...
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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

Author: koic

spec/rubocop/cop/rails/where_missing_spec.rb

@@ -255,8 +255,7 @@ def test
       RUBY
     end
 
-    it 'does not register an offense when ' \
-       '`left_joins` and `where` argument is not same relationship' do
+    it 'does not register an offense when `left_joins` and `where` argument is not same relationship' do
       expect_no_offenses(<<~RUBY)
         Foo.left_joins(:foo).where(bazs: { id: nil })
         Foo.left_joins(:foobar).where(foo: { id: nil })