PyDocs: Fix Vector.angle_signed signature

From the current signature it appears as `fallback` argument is not
optional. Changed it to have a default `None` value, similar to
`angle(other, fallback=None)`.

Noticed this issue working with fake-bpy-module that generates typing
based on Blender docs - https://github.com/nutti/fake-bpy-module/issues/326.

Pull Request: https://projects.blender.org/blender/blender/pulls/132052

source/blender/python/mathutils/mathutils_Vector.cc

@@ -1186,7 +1186,7 @@ static PyObject *Vector_angle(VectorObject *self, PyObject *args)
 PyDoc_STRVAR(
     /* Wrap. */
     Vector_angle_signed_doc,
-    ".. function:: angle_signed(other, fallback)\n"
+    ".. function:: angle_signed(other, fallback=None)\n"
     "\n"
     "   Return the signed angle between two 2D vectors (clockwise is positive).\n"
     "\n"