update book code

eBook/examples/chapter_12/read_write_file1.go

@@ -0,0 +1,21 @@
+// read_write_file.go
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+)
+
+func main() {
+	inputFile := "products.txt"
+	outputFile := "products_copy.txt"
+	buf, err := ioutil.ReadFile(inputFile)
+	if err != nil {
+		panic(err.Error())
+	}
+	fmt.Printf("%s\n", string(buf))
+	err = ioutil.WriteFile(outputFile, buf, 0x644)
+	if err != nil {
+		panic(err.Error())
+	}
+}