Unassign slash command preview is not correct when there are multiple assignees on the issue

Summary

When there are multiple assignees on the issue and /unassign slash command is used to remove one of the assignees the preview says all assignees will be removed.

The command is processed correctly and removes only the one assignee.

Steps to reproduce

1. Use an issue with multiple assignees (user1, user2)
2. Use /unassign @user1 in the issue comment
3. Click "Preview"

Example Project

I found this when commenting on https://gitlab.com/gitlab-org/gitlab-ee/issues/4185

What is the current bug behavior?

The preview says Removes assignees @user1 and @user2

What is the expected correct behavior?

The preview should say Removes assignee @user1

Relevant logs and/or screenshots

Output of checks

This bug happens on GitLab.com

/cc @victorwu @smcgivern