fix(variables6): minor typo (rust-lang#419)

Looks like this was cloned from variables5.rs

exercises/variables/variables6.rs

@@ -1,4 +1,4 @@
-// variables5.rs
+// variables6.rs
 // Make me compile! Execute the command `rustlings hint variables6` if you want a hint :)
 
 // I AM NOT DONE