bug#30941: Documentation bug.

[Fis Trivial]

On the subject: Defining Foreign Object Types
at
https://www.gnu.org/software/guile/manual/html_node/Defining-Foreign-Object-Types.html#Defining-Foreign-Object-Types

There is a example code snippet written in c. In the middle of that
snippet:

#+BEGIN_SRC c

static SCM image_type image_type;

#+END_SRC

I think the declaration is wrong, there should be only one
`image_type`. :).