Fix an incorrect auto-correct for Layout/LineLength

[koic]

This PR fixes the following incorrect auto-correct for Layout/LineLength when a heredoc is used as the first element of an array.

% cat example.rb
[<<~STRING, { key1: value1, key2: value2 }]
STRING

% ruby -c example.rb
Syntax OK

% cat .rubocop.yml
Layout/LineLength:
  Max: 40

% bundle exec rubocop --only Layout/LineLength example.rb -a
(snip)

Inspecting 1 file
C

Offenses:

example.rb:1:41: C: [Corrected] Layout/LineLength: Line is too long. [43/40]
[<<~STRING, { key1: value1, key2: value2 }]
                                        ^^^

1 file inspected, 1 offense detected, 1 offense corrected

% cat example.rb
[<<~STRING,
{ key1: value1, key2: value2 }]
STRING

% ruby -c example.rb
example.rb:1: syntax error, unexpected end-of-input, expecting ']'

---

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• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[bbatsov]

I never thought I'd see a heredoc used in an array. :-) Thanks for tackling this!