fix(move_semantics5): correct typo (rust-lang#857)

exercises/move_semantics/move_semantics5.rs

@@ -1,5 +1,5 @@
 // move_semantics5.rs
-// Make me compile only be reordering the lines in `main()`, but without
+// Make me compile only by reordering the lines in `main()`, but without
 // adding, changing or removing any of them.
 // Execute `rustlings hint move_semantics5` for hints :)