session_5: knitr fixes

session_5/session_5.Rmd

 ---
 title: "R#5: Pipping and grouping"
-author: "Laurent Modolo [laurent.modolo@ens-lyon.fr](mailto:laurent.modolo@ens-lyon.fr)
+author: "Laurent Modolo [laurent.modolo@ens-lyon.fr](mailto:laurent.modolo@ens-lyon.fr)"
 date: "2021"
 output:
   rmdformats::downcute:
@@ -361,7 +361,7 @@ Which carrier has the worst delays?
 
 <details><summary>Solution</summary>
 <p>
-```{r grouping_challenges_c, eval=F, echo = T, message=FALSE, cache=T}
+```{r grouping_challenges_c2, eval=F, echo = T, message=FALSE, cache=T}
 flights %>% 
   group_by(carrier) %>% 
   summarise(
@@ -380,7 +380,7 @@ Can you disentangle the effects of bad airports vs. bad carriers? (Hint: think a
 
 <details><summary>Solution</summary>
 <p>
-```{r grouping_challenges_c, eval=F, echo = T, message=FALSE, cache=T}
+```{r grouping_challenges_c1, eval=F, echo = T, message=FALSE, cache=T}
 flights %>% 
   group_by(carrier, dest) %>% 
   summarise(