PCV5: rename input to `img` in `pcv.spectral_index.egi`

[HaleySchuhl]

Is your feature request related to a problem? Please describe. In #1593 we updated EGI to allow for both RGB and HSI input, but renaming the parameter to the more general img would be a breaking change, so wait to update this until v5.0.

Additional context Parameter naming consistency increases usability and we try to make parameter names informative to the type of input expected.