Update ext-springs.Rmd (#307)

Changed `}` to `)`.

Author: PursuitOfDataScience

ext-springs.Rmd

@@ -164,7 +164,7 @@ We first start with the class definition:
 ```{r, eval = FALSE}
 StatSpring <- ggproto("StatSpring", Stat, 
   ...
-}
+)
 ```
 
 This creates a new `Stat`[^ext-springs-1] subclass, named `StatSpring`. ggproto classes always use CamelCase for naming, and the new class is always saved into a variable with the same name.