docs(readme): fix headings

Author: thomasleplus

README.md

@@ -69,7 +69,7 @@ docker buildx imagetools inspect leplusorg/pdf --format '{{ json (index .SBOM "l
 
 Replace `linux/amd64` by the desired platform (`linux/amd64`, `linux/arm64` etc.).
 
-### Sigstore
+## Sigstore
 
 [Sigstore](https://docs.sigstore.dev) is trying to improve supply
 chain security by allowing you to verify the origin of an