Remove repeated line and improve naming consistency

[dspinellis]

• The line psi = tbs_document.post_sign_instructions needlessly appeared twice in the code.

• The first part's output variable is used as output_handle in the second part. Make both names consistent.

Description of the changes

Small documentation fix in the interrupted signing example code.

Checklist

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• I have read the project's CoC and contribution guidelines.
• I understand and agree to the terms in the Developer Certificate of Origin as applied to this contribution.
• All new code in this PR has full test coverage.

For documentation contributions (delete if not applicable)

• I have built the HTML documentation locally, and verified that the changes behave correctly in-browser.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (7aef45a) 98.83% compared to head (df95c0b) 98.83%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #367   +/-   ##
=======================================
  Coverage   98.83%   98.83%           
=======================================
  Files         113      113           
  Lines       16289    16289           
=======================================
  Hits        16099    16099           
  Misses        190      190           

Flag	Coverage Δ
unittests	98.83% <ø> (ø)

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[MatthiasValvekens]

Thanks; will merge this.

(EDIT: I fixed the CI issue that was causing your build to fail in the meantime)

[dspinellis]

Thank you for the merge, and thank you even more for pyHanko! With it I wrote a tool to sign PDFs using a remote signature API in just a couple of days.