修正 string 练习URL

src/basic/compound-type/string-slice.md

@@ -717,7 +717,7 @@ for b in "中国人".bytes() {
 >     - [习题解答](https://github.com/sunface/rust-by-practice/blob/master/solutions/compound-types/string.md)
 > - [切片](https://practice-zh.course.rs/compound-types/slice.html)
 >     - [习题解答](https://github.com/sunface/rust-by-practice/blob/master/solutions/compound-types/slice.md)
-> - [String](https://practice-zh.course.rs/collections/String.html)
+> - [String](https://practice-zh.course.rs/collections/string.html)
 >     - [习题解答](https://github.com/sunface/rust-by-practice/blob/master/solutions/collections/String.md)
 
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