errors: add errors.Unwrap example

Change-Id: Id2336a6059f7a8d627e6c0661a4d4c05485b65f3
Reviewed-on: https://go-review.googlesource.com/c/go/+/355589
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Cherry Mui <cherryyz@google.com>
Trust: Robert Findley <rfindley@google.com>

src/errors/wrap_test.go

@@ -265,3 +265,13 @@ func ExampleAs() {
 	// Output:
 	// Failed at path: non-existing
 }
+
+func ExampleUnwrap() {
+	err1 := errors.New("error1")
+	err2 := fmt.Errorf("error2: [%w]", err1)
+	fmt.Println(err2)
+	fmt.Println(errors.Unwrap(err2))
+	// Output
+	// error2: [error1]
+	// error1
+}