Docs: Fix a grammatical error in Doc/c-api/capsule.rst (pythonGH-140577)

"who" -> "which"

Author: F18-Maverick

Doc/c-api/capsule.rst

@@ -15,7 +15,7 @@ Refer to :ref:`using-capsules` for more information on using these objects.
 .. c:type:: PyCapsule
 
    This subtype of :c:type:`PyObject` represents an opaque value, useful for C
-   extension modules who need to pass an opaque value (as a :c:expr:`void*`
+   extension modules which need to pass an opaque value (as a :c:expr:`void*`
    pointer) through Python code to other C code.  It is often used to make a C
    function pointer defined in one module available to other modules, so the
    regular import mechanism can be used to access C APIs defined in dynamically