Update check file

Author: odersky

tests/neg-custom-args/fatal-warnings/i9266.check

@@ -2,4 +2,4 @@
 3 |def test = { implicit x: Int => x + x } // error
   |                      ^
   |                      This syntax is no longer supported; parameter needs to be enclosed in (...)
-  |                      This construct can be rewritten automatically under -rewrite -source 3.0-migration.
+  |                      This construct can be rewritten automatically under -rewrite -source 3.1-migration.