Wrap the example in a code block

Wrap the code inside a code block. The declaration for ``proc even(n: int): bool`` is present twice. Hence removed one and wrapped it in a separate code block.

doc/tut1.txt

@@ -758,8 +758,7 @@ However, this cannot be done for mutually recursive procedures:
   # forward declaration:
   proc even(n: int): bool
 
-proc even(n: int): bool
-
+.. code-block:: nim
   proc odd(n: int): bool =
     assert(n >= 0) # makes sure we don't run into negative recursion
     if n == 0: false