Fix doctest warning.

doc/tutorial.rst

@@ -267,6 +267,7 @@ Additionally, for C-based languages, header definitions can be obtained via
 the :func:`loopy.generate_header`:
 
 .. doctest::
+
     >>> header = str(lp.generate_header(typed_knl)[0])
     >>> print(header)
     __kernel void __attribute__ ((reqd_work_group_size(1, 1, 1))) loopy_kernel(__global float const *__restrict__ a, int const n, __global float *__restrict__ out);