Change approvals_missing to approvals_left.

What does this MR do?

Change docs for MergeRequestApproval to update the field approvals_missing to the actual name approvals_left.

Why was this MR needed?

Documentation was wrong

Does this MR meet the acceptance criteria?

• Documentation created/updated
• Review
• Conform by the merge request performance guides
• Conform by the style guides

What are the relevant issue numbers?