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.

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extra : transplant_source : P%13%13%B8%0C%08X%B1%EE%C0%5DA%00e%60%29%E8%EC%E0%FB

docs/src/userguide/sharing_declarations.rst

@@ -165,7 +165,7 @@ 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`::