fixes package name in docs

[russbiggs]

This fixes the package name in the documentation. Currently the docs for the npm install lists the package as astro-Shield. Uppercase values are not allowed on npm, so this results in the following error:

npm ERR! code E404
npm ERR! 404 Not Found - GET https://registry.npmjs.org/@kindspells%2fastro-Shield - Not found
npm ERR! 404
npm ERR! 404  '@kindspells/astro-Shield@*' is not in this registry.
npm ERR! 404 This package name is not valid, because
npm ERR! 404  1. name can no longer contain capital letters
npm ERR! 404
npm ERR! 404 Note that you can also install from a
npm ERR! 404 tarball, folder, http url, or git url.

[castarco]

Thank you @russbiggs , I'll be merging this today.