docs: correct npm branding - use lower case

[MikeMcC399]

Description

Change text references in README and CONTRIBUTING to use lower case "npm"

Motivation and Context

https://github.com/npm/cli#faq-on-branding notes:

Is it "npm" or "NPM" or "Npm"?

npm should never be capitalized unless it is being displayed in a location that is customarily all-capitals (ex. titles on man pages).

Testing Details

N/A

Types of changes

• Documentation
• Version change (Update, remove or add more Node.js versions)
• Variant change (Update, remove or add more variants, or versions of variants)
• 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)
• Other (none of the above)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING.md document.
• All new and existing tests passed.