docs: Fix typo in help output of update command

[cmarqu]

Pull Request Check List

• A news fragment is added in news/ describing what is new.
• Test cases added for changed code.

Describe what you have changed in this PR.

Typofix in a help string.

[codecov-commenter]

Codecov Report

Base: 85.26% // Head: 85.26% // No change to project coverage 👍

Coverage data is based on head (fda1090) compared to base (d8d7343).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1615   +/-   ##
=======================================
  Coverage   85.26%   85.26%           
=======================================
  Files          92       92           
  Lines        8492     8492           
  Branches     1829     1829           
=======================================
  Hits         7241     7241           
  Misses        843      843           
  Partials      408      408           

Flag	Coverage Δ
unittests	85.09% <100.00%> (ø)

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

Impacted Files	Coverage Δ
src/pdm/cli/commands/update.py	100.00% <ø> (ø)
src/pdm/models/repositories.py	78.19% <ø> (ø)
src/pdm/cli/commands/run.py	91.59% <100.00%> (ø)
src/pdm/models/candidates.py	86.82% <100.00%> (ø)

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