Update driver documentation with the Port parameter

What does this MR do?

This change updates the driver documentation adding the "Port" parameter to the SSH section.

Why was this MR needed?

This is necessary to address the issue gitlab-org/gitlab-runner#2972 (closed).

Are there points in the code the reviewer needs to double check?

N/A

Does this MR meet the acceptance criteria?

• Documentation created/updated - yes
• Added tests for this feature/bug - N/A
• In case of conflicts with master - no conflicts

What are the relevant issue numbers?

gitlab-org/gitlab-runner#2972 (closed)