Change tp_size to tp_basicsize in comment and realign the comments (GH-6775)

Objects/unicodeobject.c

@@ -15062,7 +15062,7 @@ static PyObject *unicode_iter(PyObject *seq);
 PyTypeObject PyUnicode_Type = {
     PyVarObject_HEAD_INIT(&PyType_Type, 0)
     "str",                        /* tp_name */
-    sizeof(PyUnicodeObject),        /* tp_size */
+    sizeof(PyUnicodeObject),      /* tp_basicsize */
     0,                            /* tp_itemsize */
     /* Slots */
     (destructor)unicode_dealloc,  /* tp_dealloc */