fix(Changelog): fix _export_template variable type

[bearomorphism]

Closes #1503

Description

Checklist

• I have read the contributing guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run poetry all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (v4-9-0-test@fb04e28). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
commitizen/commands/changelog.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@              Coverage Diff               @@
##             v4-9-0-test    #1550   +/-   ##
==============================================
  Coverage               ?   97.92%           
==============================================
  Files                  ?       58           
  Lines                  ?     2698           
  Branches               ?        0           
==============================================
  Hits                   ?     2642           
  Misses                 ?       56           
  Partials               ?        0           

Flag	Coverage Δ
unittests	97.92% <85.71%> (?)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.