Reduce number of seeds in vignette

[ethanwhite]

This call in the vignette takes a little over 5 minutes to run:

lda_model_set <- LDA_set(document_term_table = rodents$document_term_table,
                         topics = c(2:6),
                         nseeds = 100,
                        control = LDA_controls_list(quiet = TRUE))

The run time is linear with nseeds so we can cut that to 30 seconds using nseeds=5 which produces pretty similar results. This seems like a good trade to me to help folks exploring the package get off to a good start, but if there's a reason to keep the 100 that's OK too.