Update README.md

[kgarigipati]

Trying to get plugin using command heroku plugins:install addons-admin in Dcoker failed with the below error

❯ heroku plugins:install addons-admin
error An unexpected error occurred: "https://registry.npmjs.org/addons-admin: Not found".
Installing plugin addons-admin... failed
Error: yarn add addons-admin@latest --non-interactive --mutex=file:/Users/kgarigipati/.local/share/heroku/yarn.lock --preferred-cache-folder=/Users/kgarigipati/Library/Caches/heroku/yarn --check-files --registry=https://registry.npmjs.org exited with code 1
    at ChildProcess.<anonymous> (/usr/local/Cellar/heroku/7.59.0/lib/client/7.62.0/node_modules/@oclif/plugin-plugins/lib/yarn.js:31:28)

This command worked heroku plugins:install @heroku-cli/plugin-addons-admin , so updating the readme

[eablack]

Thanks!