openlane/user_project_wrapper: fix ref

openlane/user_project_wrapper/config.json

diff --git a/openlane/user_project_wrapper/config.json b/openlane/user_project_wrapper/config.json
index 7fa13bb..0eb3af5 100644
--- a/openlane/user_project_wrapper/config.json
+++ b/openlane/user_project_wrapper/config.json
@@ -28,8 +28,8 @@
     "FP_SIZING": "absolute",
     "RUN_CVC": 0,
     "UNIT": 2.4,
-    "FP_IO_VEXTEND": "expr::2 * $UNIT",
-    "FP_IO_HEXTEND": "expr::2 * $UNIT",
+    "FP_IO_VEXTEND": "expr::2 * ref::$UNIT",
+    "FP_IO_HEXTEND": "expr::2 * ref::$UNIT",
     "FP_IO_VLENGTH": "ref::$UNIT",
     "FP_IO_HLENGTH": "ref::$UNIT",
     "FP_IO_VTHICKNESS_MULT": 4,