jwt: rename example to avoid vet error

After CL 603476 we were getting

jwt/example_test.go:13:1: ExampleJWTConfig refers to unknown identifier: JWTConfig

Change-Id: I51bcd06a50a852150eb6e42743431207ee00300f
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/606878
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>

jwt/example_test.go

@@ -10,7 +10,7 @@ import (
 	"golang.org/x/oauth2/jwt"
 )
 
-func ExampleJWTConfig() {
+func ExampleConfig() {
 	ctx := context.Background()
 	conf := &jwt.Config{
 		Email: "xxx@developer.com",