Gatsby: text in tutorial referrs to nvm, not npm

[chris-s-friedman]

Summary

The code block under Set default node.js version refers to nvm, not npm.

Motivation

To make the code block refer to the correct command

Draft the doc

npm install 10
npm use 10

Open a pull request

• [ ] Open a pull request with your work including the words "closes #[this issue's number]" in the pull request description

[KyleAMathews]

nvm is correct. It's a tool to let you change node versions.

https://github.com/nvm-sh/nvm

[KyleAMathews]

Thanks for contributing!

[chris-s-friedman]

oof I feel so foolish for not googling that first 🤦‍♂ Thanks!

[muescha]

this issue point out that the doc is missing a short sentence here to explain what is the nvm tool