Fix inconsistent wording in pipx install command description

[sgryjp]

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

Summary of changes

This PR changes the word PACKAGE_NAME to PACKAGE_SPEC in the description of pipx install command:

pipx install PACKAGE_NAME ...
pipx install --python PYTHON PACKAGE_NAME
...(snip)...

The PACKAGE_SPEC argument is passed directly to `pip install`.
...(snip)...

positional arguments:
  package_spec          package name(s) or pip installation spec(s)

I believe that the name of positional argument(s) of pipx install command should be package_spec since argparse is configured with that name.

Test plan

Tested by running:

$ pipx install -h | grep -c PACKAGE_NAME
0