Use explicit range when slicing into string (#2664)

Author: randomPoison

src/references/strings.md

@@ -27,7 +27,7 @@ fn main() {
     s2.push_str(s1);
     println!("s2: {s2}");
 
-    let s3: &str = &s2[s2.len() - s1.len()..];
+    let s3: &str = &s2[2..9];
     println!("s3: {s3}");
 }
 ```