Update update-pm2.md: suggest backup

[EddiesTech]

Hi!

Not thinking, I updated PM2 using a remote terminal that runs via PM2. Therefore, when I used pm2 update it started to delete all the processes for the update, and once the web server was deleted, the update process stopped and left only 3 of my processes running. I SSHed on and the pm2.dump still had all my old processes - great! Stupidly, I ran pm2 update again, which wiped them and saved the 3 processes in pm2.dump instead.

I don't think it is unreasonable to suggest a backup may be wished before updating. But, perhaps you reckon no one can be that silly when updating, in which case you can leave it out 🤣

Many thanks
Eddie