Fix rubocop offenses

```
❯ bundle exec rubocop -A
Inspecting 276 files
...................................................................................................................................................................................................................................................................................W

Offenses:

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")
                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

276 files inspected, 2 offenses detected, 2 offenses corrected
```

Author: ydah

tasks/cut_release.rake

@@ -38,7 +38,7 @@ namespace :cut_release do
   def add_header_to_changelog(version)
     update_file('CHANGELOG.md') do |changelog|
       changelog.sub("## Master (Unreleased)\n\n",
-                    '\0' "## #{version} (#{Time.now.strftime('%F')})\n\n")
+                    "\\0## #{version} (#{Time.now.strftime('%F')})\n\n")
     end
   end