fix(docs): Correct pre-commit hook id

[RoseSecurity]

🛠️ Description

• Update the README with the correct pre-commit id. If someone uses the example code listed, an error is produced:

❯ pre-commit run --all-files
[INFO] Initializing environment for https://github.com/hashicorp/copywrite.
[ERROR] `copywrite-headers` is not present in repository https://github.com/hashicorp/copywrite.  Typo? Perhaps it is introduced in a newer version?  Often `pre-commit autoupdate` fixes this.

• Remove trailing whitespace in README

👍 Definition of Done

• Simple documentation update

🤔 Can be merged upon approval?

✅

PCI review checklist

• I have documented a clear reason for, and description of, the change I am making.

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.

• If applicable, I've documented the impact of any changes to security controls.

  Examples of changes to security controls include using new access control methods, adding or removing logging pipelines, etc.

[hashicorp-cla-app]

All committers have signed the CLA.