Make error message instructional for the case of lock file being out …

…of date

Signed-off-by: Andreas Sommer <andreas.sommer87@googlemail.com>

pkg/downloader/manager.go

@@ -124,7 +124,7 @@ func (m *Manager) Build() error {
 				return errors.New("the lock file (requirements.lock) is out of sync with the dependencies file (requirements.yaml). Please update the dependencies")
 			}
 		} else {
-			return errors.New("the lock file (Chart.lock) is out of sync with the dependencies file (Chart.yaml). Please update the dependencies")
+			return errors.New("the lock file (Chart.lock) is out of sync with the dependencies file (Chart.yaml). Please update the dependencies with `helm dependency update`.")
 		}
 	}