typo in example code

Author: tshepang

src/types/closure.md

@@ -145,7 +145,7 @@ let mut u = (t, String::from("U"));
 
 let c = || {
     println!("{:?}", u); // u captured by ImmBorrow
-    u.1.truncate(0); // u.0 captured by MutBorrow
+    u.1.truncate(0); // u.1 captured by MutBorrow
     move_value(u.0.0); // u.0.0 captured by ByValue
 };
 c();