Use pipe for the spread example (#620)

To keep consistency with the rest of chapter.

tidy.Rmd

@@ -190,7 +190,8 @@ To tidy this up, we first analyse the representation in similar way to `gather()
 Once we've figured that out, we can use `spread()`, as shown programmatically below, and visually in Figure \@ref(fig:tidy-spread).
 
 ```{r}
-spread(table2, key = type, value = count)
+table2 %>%
+    spread(key = type, value = count)
 ```
 
 ```{r tidy-spread, echo = FALSE, out.width = "100%", fig.cap = "Spreading `table2` makes it tidy"}