Add max map count setting for linux to the admin tips

[edlerd]

The vm.max_map_count is a frequent problem for redis crashes on linux. It should be mentioned in the admin section of the docs.

This PR is adding the missing mention.

[netlify]

👷 Deploy request for redis-doc pending review.

Visit the deploys page to approve it

Name	Link
🔨 Latest commit	21d96d0

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}