Tweak wording to "function calls"

It's technically the same function being called and therefore run
twice.

Ref #90

examples/goroutines/goroutines.go

@@ -28,7 +28,7 @@ func main() {
         fmt.Println(msg)
     }("going")
 
-    // Our two functions are running asynchronously in
+    // Our two function calls are running asynchronously in
     // separate goroutines now, so execution falls through
     // to here. This `Scanln` code requires we press a key
     // before the program exits.