Fix an incorrect auto-correct for Layout/LineLength

[koic]

This PR fixes an incorrect auto-correct for Layout/LineLength when using heredoc as the first method argument and omitting parentheses.

% cat example.rb
foo <<~RUBY, arg
RUBY

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

Inspecting 1 file
C

Offenses:

example.rb:1:16: C: [Corrected] Layout/LineLength: Line is too long. [16/15]
foo <<~RUBY, arg
               ^

1 file inspected, 1 offense detected, 1 offense corrected

% cat example.rb
foo <<~RUBY,
arg
RUBY

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

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Before submitting the PR make sure the following are checked:

• 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.