Text suggestion for "signed_format_issue" error

[baknu]

"Signed security.txt files must start with the begin pgp signed message as the document header"

-->

"Signed security.txt must start with the header '-----BEGIN PGP SIGNED MESSAGE-----'."

See:

• https://github.com/DigitalTrustCenter/sectxt/blob/main/sectxt/__init__.py#L126
• https://github.com/DigitalTrustCenter/sectxt/blob/main/README.md

[DigitalTrustCenter]

45: Text suggestion for "signed_format_issue" error

We agree that the error message is more clear, changed as suggested