Docs: Update troubleshooting.md ulimit section Ubuntu 20.04 example

[1Const1]

• add change /etc/security/limits.conf without it changes only in system.conf&user.conf won't work
• add correct replace for all string in #DefaultLimitNOFILE=1024:524288 to DefaultLimitNOFILE=1048576:1048576 on Ubuntu 20.04

[CLAassistant]

All committers have signed the CLA.

[MichelDiz]

Can you sign the CLA?

[github-actions[bot]]

This PR has been stale for 60 days and will be closed automatically in 7 days. Comment to keep it open.