Chore fixes

[hectorj2f]

This PR attempts to fix two typos and normalize the sub-command descriptions. I hope these descriptions were not added on purpose. I found them a bit inconsistent between subcommands.

SUBCOMMANDS
  public-key         public-key gets a public key from the key-pair
  generate-key-pair  generate-key-pair generates a key-pair
  verify             Verify a signature on the supplied container image
  sign               Sign the supplied container image.
  generate           generate (unsigned) signature payloads from the supplied container image
  sign-blob          Sign the supplied blob, outputting the base64-encoded signature to stdout.
  verify-blob        Verify a signature on the supplied blob
  verify-dockerfile  Verify a signature on the base image specified in the Dockerfile
  upload             upload contains tools to upload artifacts to a registry
  download           download contains tools to download artifacts and attached artifacts in a registry
  attach             attach contains tools to attach artifacts to other artifacts in a registry

versus

SUBCOMMANDS
public-key          Gets a public key from the key-pair
generate-key-pair   Generates a key-pair
verify              Verify a signature on the supplied container image
sign                Sign the supplied container image.
attest              Attest the supplied container image.
generate            Generates (unsigned) signature payloads from the supplied container image
sign-blob           Sign the supplied blob, outputting the base64-encoded signature to stdout.
verify-attestation  Verify an attestation on the supplied container image
verify-blob         Verify a signature on the supplied blob
verify-dockerfile   Verify a signature on the base image specified in the Dockerfile
upload              Provides utilities for uploading artifacts to a registry
download            Provides utilities for downloading artifacts and attached artifacts in a registry
attach              Provides utilities for attaching artifacts to other artifacts in a registry

[dlorenc]

Thank you! This is great!

[dekkagaijin]

LGTM, please resolve the presubmit failure by signing off on your commits: https://github.com/sigstore/cosign/pull/476/checks?check_run_id=3155152516