Typo in app-graphql-playground README.md

[PoulpiFr]

Bug report

Describe the bug

Read: https://github.com/keystonejs/keystone/blob/master/packages/app-graphql-playground/README.md

in the usage example, you see new GraphQLAppPlayground(...)

while in the code: https://github.com/keystonejs/keystone/blob/master/packages/app-graphql-playground/index.js

you see GraphQLPlaygroundApp

To Reproduce

Try to use GraphQLAppPlayground as told in the usage readme.

✖ Initialising Keystone instance
TypeError: app_graphql_playground_1.GraphQLAppPlayground is not a constructor
    at Object.<anonymous> (/usr/src/app/build/entry.js:40:5)
    at Module._compile (internal/modules/cjs/loader.js:1156:30)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1176:10)
    at Module.load (internal/modules/cjs/loader.js:1000:32)
    at Function.Module._load (internal/modules/cjs/loader.js:899:14)
    at Module.require (internal/modules/cjs/loader.js:1042:19)
    at require (internal/modules/cjs/helpers.js:77:18)
    at Object.exec (/usr/src/app/node_modules/@keystonejs/keystone/bin/commands/build.js:30:58)
    at processTicksAndRejections (internal/process/task_queues.js:97:5)

Expected behaviour

The typo in the README.md should be fixed.