Update version in readme

[tlambert03]

The readme describes the qt input added in v2, but still uses v1. So it's a no-op for anyone following the readme:

Warning: Unexpected input(s) 'qt', valid inputs are ['']
Run pyvista/setup-headless-display-action@v1
  with:
    qt: true
  env:
    pythonLocation: C:\hostedtoolcache\windows\Python\3.8.10\x64
    PKG_CONFIG_PATH: C:\hostedtoolcache\windows\Python\3.8.10\x64/lib/pkgconfig
    Python_ROOT_DIR: C:\hostedtoolcache\windows\Python\3.8.10\x64
    Python2_ROOT_DIR: C:\hostedtoolcache\windows\Python\3.8.10\x64
    Python3_ROOT_DIR: C:\hostedtoolcache\windows\Python\3.8.10\x64

[larsoner]

Thanks @tlambert03 !