Fix typos in CLI help text

[SdgJlbl]

Fix #49

Successful PR Checklist:

• Tests
• Documentation (optionally: run spell checking)
• Had a good time contributing? (feel free to give some feedback)

[codecov]

Codecov Report

Merging #51 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #51   +/-   ##
=======================================
  Coverage   88.96%   88.96%           
=======================================
  Files          13       13           
  Lines         580      580           
  Branches       72       72           
=======================================
  Hits          516      516           
  Misses         49       49           
  Partials       15       15

Flag	Coverage Δ
#integration	81.2% <ø> (ø)	⬆️
#unit	56.55% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
septentrion/cli.py	79.22% <ø> (ø)	⬆️

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[mgu]

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