layers: GH1390, Fix BeginRenderPass validation msg
layers: GH1390, Fix BeginRenderPass validation msg

Incorrect index was printed in validation warning. Clarified message.

Change-Id: I77806a5fa5da25e966b009126d7a0db7a47a3908

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers