typo in examples of `melt_spectra()`

[konradmayer]

The example melt_spectra() appears to have a typo.

spectacles/R/Spectra-methods.R

Line 1148 in c5afa6f

#' p <- ggplot(r) + geom_line(aes(x=wl, y=nir, group=id, colour=fact)) + theme_bw()

The grouping variable must rather be sr_no instead of id:

p <- ggplot(r) + geom_line(aes(x=wl, y=nir, group=sr_no, colour=fact)) + theme_bw()