fixed some formatting

en/src/compound-types/string.md

@@ -89,8 +89,8 @@ fn main() {
     let s1 = String::from("hello,");
     let s2 = String::from("world!");
     let s3 = s1 + s2; 
-    assert_eq!(s3,"hello,world!");
+    assert_eq!(s3, "hello,world!");
-    println!("{}",s1);
+    println!("{}", s1);
 }
 ```
 
@@ -107,7 +107,7 @@ fn main() {
 }
 
 fn greetings(s: String) {
-    println!("{}",s)
+    println!("{}", s)
 }
 ```