updates to Installation instructions

[opkuhn]

In the Test your Installation instructions, the test for whether pyqt6 works makes reference to a datafile in pypeit/tests/files:
python -c "from importlib import resources; print(resources.files('pypeit') / 'tests/files/spec1d_r153-J0025-0312_KASTr_20150123T025323.850.fits')" | xargs -I {} pypeit_show_1dspec {}

There is no longer a files subdirectory of tests. Instead the spec1d fits file is in pypeit/data/tests.

Also, in the Developer Installation via pip, the second command, to install optional dependencies, is the same as the first.