Fix code-example syntax in documentation

The trailing ``::`` mean that the string ``:file:`volume.pxd`::`` was being treated as the code example and therefore ``cdef float cube(float)`` was simply indented as regular font.

docs/src/userguide/sharing_declarations.rst

@@ -158,7 +158,7 @@ Sharing C Functions
 
 C functions defined at the top level of a module can be made available via
 :keyword:`cimport` by putting headers for them in the ``.pxd`` file, for
-example,::
+example:
 
 :file:`volume.pxd`::