d7872a6165 changed the name of an input from `dist` to `packages-dir`,
but unfortunately it looks like GitHub actions expect underscores rather
than dashes, so deploys are currently broken with the following errors:
```
Run pypa/gh-action-pypi-publish@master
with:
user: __token__
password: ***
packages-dir: dist
env:
pythonLocation: /opt/hostedtoolcache/Python/3.8.0/x64
/usr/bin/docker run --name [...] -e INPUT_PACKAGES-DIR [...]
/app/twine-upload.sh: line 22: INPUT_PACKAGES_DIR: unbound variable
This patch replaces the dash with an underscore.
Resolves#20
