Fix linting error
Fix linting error

Make error string non-capitalized, to fix linting problem

Signed-off-by: Odin Ugedal <a12bf2c17c422acccb707fa811f07e743a9293d8@ugedal.com>

jonboulle/image-spec,jonboulle/image-spec,opencontainers/image-spec,opencontainers/image-spec