fix rsync module name in add_mirror_manager.md

[bluet]

As in mattermost Infrastructure channel, Louis Abel @Label and @NeilHanlon mentioned the rsync module name of rockylinux has been changed to rocky-linux

Author checklist (Completed by original Author)

• Good fit for the Rocky Linux project? Title and Author Metatags inserted ?
• If applicable, steps and instructions have been tested to work
• Initial self-review to fix basic typos and grammar completed

Rocky Documentation checklist (Completed by Rocky team)

• 1st Pass (Document is good fit for project and author checklist completed)
• 2nd Pass (Technical Review - check for technical correctness)
• 3rd Pass (Detailed Editorial Review and Peer Review)
• Final approval (Final Review)

[github-actions]

Test results for efd9d32:

Number of broken URLs: 4

URL,RESULT,FILENAME
 https://ftp.gnu.org/gnu/hello/hello-2.12.tar.gz,failed,labs/systems_administration_I/lab7-software_management.md
 http://your_ip,failed,guides/cms/mediawiki.md
 http://$(hostname):8080,failed,guides/repositories/pulp_fetch_upload.md
 http://howtotechwriting.com/,failed,guides/contribute/README.md