fix expression label

wizard/expression_builder.py

@@ -17,7 +17,7 @@
     # when the wizard action is completed, this field is
     # written in the substitution value
     expression = fields.Char(
-        string="Substitution",
+        string="Expression",
         help=(
             "Stores the substitution value the builder is working on."
             " (example: 'partner_id.name')"