append vertical spacing to message

delineates the message from the model printer a bit better:

```
> C5_rules()
parsnip could not locate an implementation for `C5_rules` classification model specifications using the `C5.0` engine.
ℹ The parsnip extension package rules implements support for this specification.
ℹ Please install (if needed) and load to continue.

C5.0 Model Specification (classification)

Computational engine: C5.0
```

Author: simonpcouch

R/misc.R

@@ -140,7 +140,8 @@ prompt_missing_implementation <- function(spec_, engine_, mode_) {
         msg,
         i = paste0("The parsnip extension package ", all$pkg[[1]],
                    " implements support for this specification."),
-        i = "Please install (if needed) and load to continue."
+        i = "Please install (if needed) and load to continue.",
+        ""
       )
   }