Update README.md to fix two small issues in runner registration section

What does this MR do?

• fixes an error in the gitlab runner doc
• updates an example in this doc to be less confusing

Why was this MR needed?

• the note in the doc was in contradiction to the command line argument usage in the example

What's the best way to test this MR?

• view the updated readme

What are the relevant issue numbers?

n.a.