cli: fix usage string in help command

[felicianotech]

• Have you followed the guidelines for contributing?
• Have you signed the CLA?
• If this is a bugfix. Have you checked that there is a bug report open for the issue you are trying to fix on bug reports?
• If this is a new feature. Have you discussed the design on the forum?
• Have you successfully run ./runtests.sh static?
• Have you successfully run ./runtests.sh tests/unit?

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LP: https://bugs.launchpad.net/snapcraft/+bug/1745040
Forum: https://forum.snapcraft.io/t/snapcraft-help-command-showing-wrong-usage/9045?u=felicianotech

P.S. The last two items in the checklist above, aren't documented well in the HACKING guide and don't even pass on master.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@e9ae8d2). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master    #2429   +/-   ##
=========================================
  Coverage          ?   90.02%           
=========================================
  Files             ?      196           
  Lines             ?    12902           
  Branches          ?     1959           
=========================================
  Hits              ?    11615           
  Misses            ?      886           
  Partials          ?      401

Impacted Files	Coverage Δ
snapcraft/cli/help.py	90.69% <100%> (ø)

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[sergiusens]

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