Fix a build error

This commit fixes the following build error.

```console
% bundle exec rake
(snip)

spec/support/multiline_literal_brace_layout_trailing_comma_examples.rb:14:5:
C: [Correctable] Layout/LineEndStringConcatenationIndentation: Indent
the first part of a string concatenated with backslash.
'opening [...]'
^^^^^^^^^^^^^^^

1289 files inspected, 1 offense detected, 1 offense auto-correctable
```

https://github.com/rubocop/rubocop/runs/2892303101

spec/support/multiline_literal_brace_layout_trailing_comma_examples.rb

@@ -10,8 +10,7 @@
 
   # same line message for symmetrical style (use [...] to abbreviate).
   let(:same_line_message) do
-    'Closing brace must be on the same line as the last element when ' \
-    'opening [...]'
+    'Closing brace must be on the same line as the last element when opening [...]'
   end
 
   # same line message for same_line style