Formatting some sample code (#529)

* Update 01.2.md

删除：
- 不支持断言

* Formatting 问题5.1 code

* Formatting 问题5.3 code

* Correct the translation

- correct `令 v := map1[key1] 可以将 key1 对应的值赋值为 v` to `令 v := map1[key1] 可以将 key1 对应的值赋值给 v`

eBook/05.6.md

@@ -87,7 +87,7 @@ for { //since there are no checks, this is an infinite loop
 	if i >= 3 { break }
 	//break out of this for loop when this condition is met
 	fmt.Println("Value of i is:", i)
-	i++;
+	i++
 }
 fmt.Println("A statement just after for loop.")
 ```