Uses deprecated mutate_() function from dplyr

[DarkShinyLugia]

When attempting to knit "eda_workshop_student.rmd" from the associated hst953 class, this package will conistently throw the following error when used on R Studio build 242:

processing file: eda_workshop_student.Rmd
  |...................................                |  69% [unnamed-chunk-23]

Error:
! `mutate_()` was deprecated in dplyr 0.7.0 and is now defunct.
ℹ Please use `mutate()` instead.
ℹ See vignette('programming') for more help
Backtrace:
    ▆
 1. ├─MIMICbook::plot_prop_by_level(dat2, "sofa.cat", "hosp_exp_flg")
 2. │ └─... %>% ...
 3. ├─dplyr::summarise(...)
 4. └─dplyr::mutate_(...)
 5.   └─dplyr:::lazy_defunct("mutate")
 6.     └─lifecycle::deprecate_stop(...)
 7.       └─lifecycle:::deprecate_stop0(msg)
 8.         └─rlang::cnd_signal(...)
                                                                                                             

Quitting from eda_workshop_student.Rmd:310-312 [unnamed-chunk-23]
Execution halted

This prevents users that are using the most recent dplyr package from completing this course, and is a general headache to deal with