Issue #21440: Small fix to curl example for adding member to project

## What does this MR do?
Two lines changed in docs to move the user_id field out of the url and onto the query string 

## Why was this MR needed?
Issue #21440 reported under Issues. The documentation example for adding members to a project had a slight problem. 

## What are the relevant issue numbers?
Closes #21440.

See merge request !6236