Fix Position docs (#3223)

Author: flying-sheep

R/position-.r

@@ -17,7 +17,7 @@
 #'     per layer. `panel` is currently an internal data structure, so
 #'     this method should not be overridden.
 #'
-#'   - `compute_panel(self, data, params, panel)` is called once per
+#'   - `compute_panel(self, data, params, scales)` is called once per
 #'     panel and should return a modified data frame.
 #'
 #'     `data` is a data frame containing the variables named according