Remove [sudo] for Yarn install/upgrade command in README.md
Remove [sudo] for Yarn install/upgrade command in README.md

Fixes https://github.com/thelounge/thelounge/pull/2664#discussion_r203621102
FryDay/lounge,ScoutLink/lounge,MaxLeiter/lounge,williamboman/lounge,FryDay/lounge,MaxLeiter/lounge,FryDay/lounge,ScoutLink/lounge,thelounge/lounge,williamboman/lounge,williamboman/lounge,thelounge/lounge,MaxLeiter/lounge,ScoutLink/lounge