Fix typo

Author: mo8it

exercises/move_semantics/move_semantics3.rs

@@ -1,6 +1,6 @@
 // move_semantics3.rs
 //
-// Make me compile without adding new lines-- just changing existing lines! (no
+// Make me compile without adding new lines -- just changing existing lines! (no
 // lines with multiple semicolons necessary!)
 //
 // Execute `rustlings hint move_semantics3` or use the `hint` watch subcommand