Fix typos (#3646)

docs/getting-started/configure-app-with-plugins.md

@@ -11,7 +11,7 @@ add an existing plugin to it. We are using the
 [CircleCI](https://github.com/backstage/backstage/blob/master/plugins/circleci/README.md)
 plugin in this example.
 
-1. Add the plugin's NPM package to the repo:
+1. Add the plugin's npm package to the repo:
 
 ```bash
 yarn add @backstage/plugin-circleci

docs/getting-started/create-an-app.md

@@ -87,7 +87,7 @@ You may encounter the following error message:
 Couldn't find any versions for "file-saver" that matches "eligrey-FileSaver.js-1.3.8.tar.gz-art-external"
 ```
 
-This is likely because you have a globally configured NPM proxy, which breaks
+This is likely because you have a globally configured npm proxy, which breaks
 the installation of the `material-table` dependency. This is a known issue and
 being worked on in `material-table`, but for now you can work around it using
 the following:

docs/getting-started/index.md

@@ -10,7 +10,7 @@ you're planning to do.
 
 Creating a standalone instance makes it simpler to customize the application for
 your needs whilst staying up to date with the project. You will also depend on
-`@backstage` packages from NPM, making the project much smaller. This is the
+`@backstage` packages from npm, making the project much smaller. This is the
 recommended approach if you want to kick the tyres of Backstage or setup your
 own instance.