Update README.md

[hefemk]

Description

Update the description in README:

• According to the installation instructions on npm, the correct command should be npm install @adobe/cgroup-metrics
• cgroup.cpu() should actually be cgroup.cpu
• cgroup.memory() should actually be cgroup.memory

Related Issue

https://github.com/adobe/node-cgroup-metrics/issues/56

Motivation and Context

To prevent users from experiencing installation or usage difficulties.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] I have signed the Adobe Open Source CLA.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.