Fix wrong error message for lower percent i arrays

Not so sure how to trigger it but this is definitly more correct.

Author: Earlopain

src/prism.c

@@ -19343,7 +19343,7 @@ parse_expression_prefix(pm_parser_t *parser, pm_binding_power_t binding_power, b
                     pm_array_node_elements_append(array, current);
                     current = NULL;
                 } else {
-                    expect1(parser, PM_TOKEN_STRING_CONTENT, PM_ERR_LIST_W_LOWER_ELEMENT);
+                    expect1(parser, PM_TOKEN_STRING_CONTENT, PM_ERR_LIST_I_LOWER_ELEMENT);
                 }
             }