Make npm lowercase in the GitLab docs

Problem to solve

npm is a package manager for the JavaScript programming language. npm, Inc. is a subsidiary of GitHub, that provides hosting for software development and version control with the usage of Git.

In GitLab's documentation, npm is often represented in all caps as NPM. I believe this is incorrect and should be updated to be all lowercase.

Proposal

Update NPM to npm in the Package Registry documentation, so that we accurately represent the product and company.