Fix the package name in readme

[taras]

Motivation

I'm trying to install @shipshapecode/backstage-plugin-graphql-voyager by following the readme, but I'm getting an error that package is not available

➜  app git:(graphql-plugin) yarn add @shipshapecode/plugin-graphql-voyager
yarn add v1.22.10
[1/4] 🔍  Resolving packages...
error An unexpected error occurred: "https://registry.yarnpkg.com/@shipshapecode%2fplugin-graphql-voyager: Not found".
info If you think this is a bug, please open a bug report with the information provided in "/Users/tarasmankovski/Repositories/backstage-demo/packages/app/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.

Approach

Fix the name of the package in the readme.