Docs Fix: Add in the required type for the content part

[internalsystemerror]

The current documentation is missing this crucial requirement for the multipart/alternative + attachments section.

This had me stumped for a long time and I finally found the solution here:
https://akrabat.com/sending-attachments-in-multipart-emails-with-zendmail/

Note that the \n character is not necessary (and I think in some clients causes errors).

[weierophinney]

This repository has been closed and moved to laminas/laminas-mail; a new issue has been opened at laminas/laminas-mail#16.

[weierophinney]

This repository has been moved to laminas/laminas-mail. If you feel that this patch is still relevant, please re-open against that repository, and reference this issue. To re-open, we suggest the following workflow:

• Squash all commits in your branch (git rebase -i origin/{branch})
• Make a note of all changed files (`git diff --name-only origin/{branch}...HEAD
• Run the laminas/laminas-migration tool on the code.
• Clone laminas/laminas-mail to another directory.
• Copy the files from the second bullet point to the clone of laminas/laminas-mail.
• In your clone of laminas/laminas-mail, commit the files, push to your fork, and open the new PR.
  We will be providing tooling via laminas/laminas-migration soon to help automate the process.