Update 1Password instructions in the "Security" page of Handbook

I'm following the onboarding instructions and the "Security" page in the Handbook and I believe that there were some changes on the 1Password side. I don't have any previous experience with 1Password, but I think that now they treat your personal account as a team account with domain set to https://my.1password.com Also, some wording in the application has changed. I'll try to explore it and improve the instructions.

/cc @rspeicher