fix README file to use correct builder instance

[kizzx2]

The default builder name default does not match the builder that is created by the setup-buildx-action, leading to cache not being used

[AkihiroSuda]

Thanks, but please sign the commit for DCO
https://github.com/apps/dco

(run git commit -a -s --amend, and make sure that the Signed-off-by: NAME <EMAIL> line with your real name is included in the commit message)

[kizzx2]

@AkihiroSuda Sure done please check

[AkihiroSuda]

Thanks