change minor mistake of spelling

Signed-off-by: allencloud <allen.sun@daocloud.io>

node_inspect_test.go

@@ -31,7 +31,7 @@ func TestNodeInspectNodeNotFound(t *testing.T) {
 
 	_, _, err := client.NodeInspectWithRaw(context.Background(), "unknown")
 	if err == nil || !IsErrNodeNotFound(err) {
-		t.Fatalf("expected an nodeNotFoundError error, got %v", err)
+		t.Fatalf("expected a nodeNotFoundError error, got %v", err)
 	}
 }