replace EpiNow2::dist_spec() with EpiNow2::pmf() when appropriate

[avallecam]

This is a reminder to suggest that after epiforecasts/EpiNow2#504 gets merged, line 17 could be updated following the chunk below:

episoap/inst/rmarkdown/templates/transmissibility/skeleton/rmdchunks/EpiNow2.Rmd

Lines 16 to 18 in 8f2d861

	generation_time <- generation_time_opts(
	dist_spec(pmf = si$prob_dist$d(si_x))
	)

 generation_time <-  generation_time_opts( 
   EpiNow2::pmf(si$prob_dist$d(si_x))
 ) 

This came after some exploration shared in #130

[CarmenTamayo]

thanks @avallecam for the heads up!