include parens in function name

Co-authored-by: Hugo Gruson <hugo.gruson@protonmail.com>

Author: maneesha

episodes/02-func-R.Rmd

@@ -266,7 +266,7 @@ z
 center(z, 3)
 ```
 
-That looks right, so let's try `center` on our real data.
+That looks right, so let's try `center()` on our real data.
 We'll center the inflammation data from day 4 around 0:
 
 ```{r}