fix: put sub-command text as part of positional argument help text

[KelvinChung2000]

close #1715

Before:

Usage: base [-h] SUBCOMMAND ...

Manage the widgets.

Options:
  -h, --help   show this help message and exit

  SUBCOMMAND
    add        add a widget
    list (ls)  list widgets

After:

Usage: base [-h] SUBCOMMAND ...

Manage the widgets.

Positional Arguments:
  SUBCOMMAND
    add        add a widget
    list (ls)  list widgets

Options:
  -h, --help   show this help message and exit

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.57%. Comparing base (cb4e6c5) to head (cde81ec).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1719      +/-   ##
==========================================
- Coverage   99.61%   99.57%   -0.04%     
==========================================
  Files          23       23              
  Lines        5906     5910       +4     
==========================================
+ Hits         5883     5885       +2     
- Misses         23       25       +2     

Flag	Coverage Δ
unittests	99.57% <100.00%> (-0.04%)	⬇️

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