Update the example in `add_p.tbl_continuous()` to use more modifications

[ddsjoberg]

The only example in add_p.tbl_continuous() uses all the defaults, which can make it somewhat less clear how to change the defaults. I think we could update the example or add another.

https://www.danieldsjoberg.com/gtsummary/reference/add_p.tbl_continuous.html#ref-examples