Change duplicated chunk label

It's helpful to prefix chunks with something identifying the chapter. This chunk label was used in a previous chapter, evidently. This issue will go away when we update the slides to use Quarto, so I'm not making a big deal out of it!

21_fifa-19.Rmd

@@ -375,7 +375,7 @@ plot(fifa_mp_gbm_shallow, fifa_mp_gbm_deep, fifa_mp_rf, fifa_mp_rm,
 
 **Partial Dependence Profiles**  
 
-```{r pdp}
+```{r fifa-pdp}
 selected_variables <- c("movement_reactions", "skill_ball_control", "skill_dribbling", "age")
 
 fifa19_pd_shallow <- model_profile(fifa_gbm_exp_shallow,