Display help message when pipx install is run without arguments

[dukecat0]

• I have added a news fragment under changelog.d/ (if the patch affects the end users)

Summary of changes

Another minor fix.

Original behavior:

$ pipx install
$

New behavior:

$ pipx install
usage: pipx install [-h] [--quiet] [--verbose] [--include-deps] [--force] [--suffix SUFFIX] [--python PYTHON] [--fetch-missing-python]
                    [--preinstall PREINSTALL] [--system-site-packages] [--index-url INDEX_URL] [--editable] [--pip-args PIP_ARGS]
                    package_spec [package_spec ...]
pipx install: error: the following arguments are required: package_spec

Test plan

Tested by running

pipx install