fix(issue #762): correct error message when updating service

* correct message when updating service and passing many names
* fix issue with TestServiceUpdateWithMultipleImages running create vs update

CHANGELOG.adoc

@@ -26,6 +26,10 @@
 | Fix plugin lookup with file ext on Windows
 | https://github.com/knative/client/pull/774[#774]
 
+| 🐛
+| correct error message when updating service
+| https://github.com/knative/client/pull/778[#778]
+
 |===
 
 ## v0.13.1 (2020-03-25)

pkg/kn/commands/service/update.go

@@ -60,7 +60,7 @@ func NewServiceUpdateCommand(p *commands.KnParams) *cobra.Command {
 		Example: updateExample,
 		RunE: func(cmd *cobra.Command, args []string) (err error) {
 			if len(args) != 1 {
-				return errors.New("requires the service name")
+				return errors.New("'service update' requires the service name given as single argument")
 			}
 
 			namespace, err := p.GetNamespace(cmd)

pkg/kn/commands/service/update_test.go

@@ -195,11 +195,19 @@ func TestServiceUpdateImage(t *testing.T) {
 func TestServiceUpdateWithMultipleImages(t *testing.T) {
 	orig := newEmptyService()
 	_, _, _, err := fakeServiceUpdate(orig, []string{
-		"service", "create", "foo", "--image", "gcr.io/foo/bar:baz", "--image", "gcr.io/bar/foo:baz", "--no-wait"})
+		"service", "update", "foo", "--image", "gcr.io/foo/bar:baz", "--image", "gcr.io/bar/foo:baz", "--no-wait"})
 
 	assert.Assert(t, util.ContainsAll(err.Error(), "\"--image\"", "\"gcr.io/bar/foo:baz\"", "flag", "once"))
 }
 
+func TestServiceUpdateWithMultipleNames(t *testing.T) {
+	orig := newEmptyService()
+	_, _, _, err := fakeServiceUpdate(orig, []string{
+		"service", "update", "foo", "foo1", "--image", "gcr.io/foo/bar:baz", "--no-wait"})
+
+	assert.Assert(t, util.ContainsAll(err.Error(), "'service update' requires the service name given as single argument"))
+}
+
 func TestServiceUpdateCommand(t *testing.T) {
 	orig := newEmptyService()