Update entry.rb to extract NtryRef

[Mant-Ve]

• Added the ntry_ref method in entry.rb to ensure correct extraction of NtryRef.
• Added an RSpec test (spec/camt_parser_entry_spec.rb) to validate the new behavior.

[tobischo]

Thank you for your contribution.

I left a comment about a change that should be made before I would consider merging it

[frantisekrokusek]

@tobischo I think we can close this one 😜

[tobischo]

Indeed, covered in #71