bug#12241: errata in section 2.4 of Guile v2.0.6 manual

[Kurt W. Gochko]

The function j0_wrapper() in section 2.4 Writing Guile Extensions of
version 2.0.6 of the Guile Reference manual is incorrect.
        return scm_make_real (j0 (scm_num2dbl (x, "j0")));
should instead be
        return scm_from_double (j0 (scm_to_double (x)));
Note that the correct function can be found in section 5.3.1
A Simple Guile Extension.