update book code

eBook/examples/chapter_6/function_parameter.go

@@ -0,0 +1,20 @@
+// function_parameter.go
+package main
+
+import (
+	"fmt"
+)
+
+func main() {
+	callback(1, Add)
+}
+
+func Add(a,b int) {
+	fmt.Printf("The sum of %d and %d is: %d\n", a, b, a + b)
+}
+
+
+func callback(y int, f func(int, int)) {
+	f(y, 2)  // this becomes Add(1, 2)
+}
+// Output:  The sum of 1 and 2 is: 3