doc: update notation sign and verify spec for metadata (notaryproject…

…#498)

Spec update to support notaryproject/roadmap#67

`notation sign`:
- user will be able to specify additional key value pairs with the `--user-metadata` flag (`-um` short) that will be signed as part of the payload.

`notation verify`:
- user will be able to specify additional key value pairs with the `--user-metadata` flag (`-um` short) that must be present in the signature to pass verification.

Signed-off-by: Byron Chien <chienb@amazon.com>

specs/commandline/sign.md

@@ -114,7 +114,7 @@ notation sign <registry>/<repository>@<digest>
 ### Sign an OCI Artifact with user metadata
 
 ```shell
-# Prerequisites: 
+# Prerequisites:
 # A default signing key is configured using CLI "notation key"
 
 # sign an artifact stored in a registry and add user-metadata io.wabbit-networks.buildId=123 to the payload
@@ -165,7 +165,7 @@ notation sign --image-spec v1.1-image <registry>/<repository>@<digest>
 ```
 
 [oci-artifact-manifest]: https://github.com/opencontainers/image-spec/blob/v1.1.0-rc2/artifact.md
-[oci-image-spec]: https://github.com/opencontainers/image-spec/blob/v1.1.0-rc2/spec.md 
+[oci-image-spec]: https://github.com/opencontainers/image-spec/blob/v1.1.0-rc2/spec.md
 [oci-backward-compatibility]: https://github.com/opencontainers/distribution-spec/blob/v1.1.0-rc1/spec.md#backwards-compatibility
 [registry-support]: https://notaryproject.dev/docs/registrysupport/
 [oras-land]: https://oras.land/