More helpful error message when not sufficiently argumentative

The error message suggests you need one argument even when you
have provided one. Suggest having another argument.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>
Upstream-commit: 4ce93940b7c2a640c94296f20352cf6f052d1028
Component: engine

components/engine/builder/dockerfile/evaluator_test.go

@@ -28,7 +28,7 @@ func initDispatchTestCases() []dispatchTestCase {
 		dockerfile: `COPY
 	quux \
       bar`,
-		expectedError: "COPY requires at least one argument",
+		expectedError: "COPY requires at least two arguments",
 	},
 		{
 			name:          "ONBUILD forbidden FROM",