docs: fix sig-proxy flag spelling

[MukundaKatta]

Summary

• fix the run command reference to document --sig-proxy with the correct long flag spelling

Testing

• git diff --check

[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)