cue: fix Value.Subsumes example to correct function naming convention

go vet reported:
  ExampleSubsumes refers to unknown identifier: Subsumes

Fix ExampleSubsumes func name with adding "Value_" suffix before Subsumes.

Change-Id: Ic368e7a81a354bb000f796b7d3c4e4105ec40ddc
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/4541
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/cue/examples_test.go b/cue/examples_test.go
index 7505f3b..be46bbd 100644
--- a/cue/examples_test.go
+++ b/cue/examples_test.go
@@ -66,7 +66,7 @@
 	// json: cannot unmarshal string into Go struct field ab.B of type int
 }
 
-func ExampleSubsumes() {
+func ExampleValue_Subsumes() {
 	// Check compatibility of successive APIs.
 	var r cue.Runtime