Change sample IP address from 1.1.1.1 to 198.51.100.1

See the general Documentation guidelines: https://docs.gitlab.com/ee/development/documentation/.

What does this MR do?

The sample IP address is changed from 1.1.1.1 to an actual documentation IP. 1.1.1.1 is now in use by Cloudflare and should therefore no longer be used in any kind of documentation. 198.51.100.1 is part of the 198.51.100.0/24 TEST-NET-2 reserved by the IETF for documentation purposes. Using TEST-NET-2 instead of TEST-NET-1 to prevent confusion as TEST-NET-1 has the same 192. prefix as the 192.168. local subnet.