extmod/modbtree: Rename "sync" method to "flush" for consistency.

Rename recently introduced "sync" method to "flush" for consistency with
usual files.

extmod/modbtree.c

@@ -81,11 +81,11 @@ STATIC void btree_print(const mp_print_t *print, mp_obj_t self_in, mp_print_kind
     mp_printf(print, "<btree %p>", self->db);
 }
 
-STATIC mp_obj_t btree_sync(mp_obj_t self_in) {
+STATIC mp_obj_t btree_flush(mp_obj_t self_in) {
     mp_obj_btree_t *self = MP_OBJ_TO_PTR(self_in);
     return MP_OBJ_NEW_SMALL_INT(__bt_sync(self->db, 0));
 }
-STATIC MP_DEFINE_CONST_FUN_OBJ_1(btree_sync_obj, btree_sync);
+STATIC MP_DEFINE_CONST_FUN_OBJ_1(btree_flush_obj, btree_flush);
 
 STATIC mp_obj_t btree_close(mp_obj_t self_in) {
     mp_obj_btree_t *self = MP_OBJ_TO_PTR(self_in);
@@ -320,7 +320,7 @@ STATIC mp_obj_t btree_binary_op(mp_uint_t op, mp_obj_t lhs_in, mp_obj_t rhs_in)
 
 STATIC const mp_rom_map_elem_t btree_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_close), MP_ROM_PTR(&btree_close_obj) },
-    { MP_ROM_QSTR(MP_QSTR_sync), MP_ROM_PTR(&btree_sync_obj) },
+    { MP_ROM_QSTR(MP_QSTR_flush), MP_ROM_PTR(&btree_flush_obj) },
     { MP_ROM_QSTR(MP_QSTR_get), MP_ROM_PTR(&btree_get_obj) },
     { MP_ROM_QSTR(MP_QSTR_put), MP_ROM_PTR(&btree_put_obj) },
     { MP_ROM_QSTR(MP_QSTR_seq), MP_ROM_PTR(&btree_seq_obj) },