Fix two minor issues in the tutorial.

Remove an extraneous 'then', and replace 'second' with 'first'.

doc/tutorial.md

@@ -226,9 +226,9 @@ let monster_size: int = 50;
 ~~~~
 
 Local variables may shadow earlier declarations, as in the previous example:
-`monster_size` was first declared as a `float`, and then then a second
+`monster_size` was first declared as a `float`, and then a second
 `monster_size` was declared as an int. If you were to actually compile this
-example, though, the compiler will determine that the second `monster_size` is
+example, though, the compiler will determine that the first `monster_size` is
 unused and issue a warning (because this situation is likely to indicate a
 programmer error). For occasions where unused variables are intentional, their
 name may be prefixed with an underscore to silence the warning, like `let