Cleanup: Message bus, removed incorrect return type from docstring

The docstring was likely copy-pasted, and the return type was not adjusted.
Since the `bpy.msgbus.subscribe_rna()` returns nothing, I just removed the
return type documentation.

source/blender/python/intern/bpy_msgbus.c

@@ -214,9 +214,7 @@ PyDoc_STRVAR(
     "\n"
     "      - ``PERSISTENT`` when set, the subscriber will be kept when remapping ID data.\n"
     "\n"
-    "   :type options: set of str.\n"
-    "\n"
-    "   Returns a new vector int property definition.\n");
+    "   :type options: set of str.\n");
 static PyObject *bpy_msgbus_subscribe_rna(PyObject *UNUSED(self), PyObject *args, PyObject *kw)
 {
   const char *error_prefix = "subscribe_rna";