Replace `n_demo_grps` object name across vignettes

[avallecam]

This suggestion is minor, but would improve the readability and help differentiate it from the demo_grp column from final_size() output.

Given that the number of susceptible groups are assigned to n_susc_groups, the number of demographic groups could be assigned to n_demo_groups:

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/finalsize.Rmd#L139-L143

This would require modification in the following lines:

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/finalsize.Rmd#L114

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/finalsize.Rmd#L141

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/finalsize.Rmd#L150

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/finalsize.Rmd#L155

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/varying_susceptibility.Rmd#L89

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/varying_susceptibility.Rmd#L120

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/varying_susceptibility.Rmd#L141

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/varying_susceptibility.Rmd#L257

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/uncertainty_params.Rmd#L87

https://github.com/epiverse-trace/finalsize/blob/f0ac4127e54579754d08ee92754203f99c0f6ee9/vignettes/uncertainty_params.Rmd#L101

[pratikunterwegs]

I think users will understand that the n_ prefix indicates a count, this does not seem important enough to change.