Fix typo in allow_password_change help

[manics]

I spotted a typo in the help output for allow_password_change

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[codecov-commenter]

Codecov Report

Merging #559 (b151479) into master (05913dd) will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #559      +/-   ##
==========================================
- Coverage   76.99%   76.95%   -0.05%     
==========================================
  Files         109      109              
  Lines        9948     9948              
  Branches     1078     1078              
==========================================
- Hits         7659     7655       -4     
- Misses       1909     1912       +3     
- Partials      380      381       +1     

Impacted Files	Coverage Δ
jupyter_server/serverapp.py	66.10% <ø> (ø)
jupyter_server/services/kernels/handlers.py	61.37% <0.00%> (-1.59%)	⬇️
jupyter_server/tests/services/kernels/test_api.py	99.01% <0.00%> (+1.96%)	⬆️

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[kevin-bates]

Thank you!

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