Clarified "Team Name" in Migrate from Slack section

[AlexKalopsia]

Summary

The documentation simply says that <TEAM-NAME> needed to be a single word.
My team called "Team" is one word, so I just used that, which caused failure during the migration.

I eventually found out (via using mmctl team list --json) that I was using the display_name ("Team") instead of name ("team").
I have clarified the documentation to prevent other users from running into the same issue.

[mattermost-build]

Hello @AlexKalopsia,

Thanks for your pull request! A Core Committer will review your pull request soon. For code contributions, you can learn more about the review process here.

Per the Mattermost Contribution Guide, we need to add you to the list of approved contributors for the Mattermost project.

Please help complete the Mattermost contribution license agreement?
Once you have signed the CLA, please comment with /check-cla and confirm that the CLA check is green.

This is a standard procedure for many open source projects.

Please let us know if you have any questions.

We are very happy to have you join our growing community! If you're not yet a member, please consider joining our Contributors community channel to meet other contributors and discuss new opportunities with the core team.

[github-actions]

Newest code from AlexKalopsia has been published to preview environment for Git SHA 92d97cf

[cwarnermm]

Thank you, @AlexKalopsia!! Very much appreciated!

[cwarnermm]

@AlexKalopsia - I'll merge this correction as soon as an upstream issue with CLA checks is resolved.

[mattermost-build]

This PR has been automatically labelled "stale" because it hasn't had recent activity.
A core team member will check in on the status of the PR to help with questions.
Thank you for your contribution!