Warning message in `add_risktable_strata_symbol()` forwards user to next warning message

[timtreis]

When calling add_risktable_strata_symbol() on a ggsurvfit object, it tells the user to call add_risktable() beforehand.

However, an inexperienced user will just add + add_risktable() to the command, which will inevitably trigger the next warning since it has to be called as + add_risktable(risktable_group='risktable_stats').

This is a tiny bit annoying and could be streamlined, by, for example, just including the required argument in the first thrown warning.

[ddsjoberg]

I like the suggestion! We'll get to it soon :)