update book code

eBook/examples/chapter_8/map_func.go

@@ -1,11 +1,13 @@
-package main
-import "fmt"
-
-func main() {
-    mf := map[int]func() int{
-        1: func() int { return 10 },
-        2: func() int { return 20 },
-        5: func() int { return 50 },
-    }
-    fmt.Println(mf)
-}
+// map_func.go
+package main
+import "fmt"
+
+func main() {
+	mf := map[int]func() int{
+		1: func() int { return 10 },
+		2: func() int { return 20 },
+		5: func() int { return 50 },
+	}
+	fmt.Println(mf)
+}
+// Output:  map[1:0x10903be0 5:0x10903ba0 2:0x10903bc0]