py: Fix prefix on few sequence helpers, was incorrectly "mp_".
diff --git a/py/objstr.c b/py/objstr.c
index 26c6edd..80b0cf8 100644
--- a/py/objstr.c
+++ b/py/objstr.c
@@ -333,7 +333,7 @@
 #if MICROPY_ENABLE_SLICE
         if (MP_OBJ_IS_TYPE(index, &mp_type_slice)) {
             machine_uint_t start, stop;
-            if (!m_seq_get_fast_slice_indexes(self_len, index, &start, &stop)) {
+            if (!mp_seq_get_fast_slice_indexes(self_len, index, &start, &stop)) {
                 assert(0);
             }
             return mp_obj_new_str(self_data + start, stop - start, false);