Fix a rubocop offense

[ydah]

This PR fixes rubocop offense

tasks/cut_release.rake:41:21: W: [Corrected] Lint/ImplicitStringConcatenation: Combine '\0' and "## #{version} (#{Time.now.strftime('%F')})\n\n" into a single string literal, rather than using implicit string concatenation. Or, if they were intended to be separate method arguments, separate them with a comma.
                    '\0' "## #{version} (#{Time.now.strftime('%F')})\n\n")
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
tasks/cut_release.rake:41:21: C: [Corrected] Style/StringConcatenation: Prefer string interpolation to string concatenation.
                    '\0' + "## #{version} (#{Time.now.strftime('%F')})\n\n")
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• [-] Added tests.
• [-] Updated documentation.
• [-] Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).