修改defer关闭文件代码片段中的错误 (#403)

eBook/12.7.md

@@ -4,10 +4,10 @@
 
 ```go
 func data(name string) string {
-	f := os.Open(name, os.O_RDONLY, 0)
+	f, _ := os.OpenFile(name, os.O_RDONLY, 0)
 	defer f.Close() // idiomatic Go code!
-	contents := io.ReadAll(f)
-	return contents
+	contents, _ := ioutil.ReadAll(f)
+	return string(contents)
 }
 ```