NPM -> npm [ci skip]

According to https://www.npmjs.com/.

RELEASING_RAILS.md

@@ -125,7 +125,7 @@ guide. You can generate keys with the GPG suite from here: https://gpgtools.org.
 Run `rake changelog:header` to put a header with the new version in every
 CHANGELOG. Don't commit this, the release task handles it.
 
-Run `rake release`. This will populate the gemspecs and NPM package.json with
+Run `rake release`. This will populate the gemspecs and npm package.json with
 the current RAILS_VERSION, commit the changes, tag it, and push the gems to
 rubygems.org.
 

actionview/app/assets/javascripts/README.md

@@ -15,7 +15,7 @@ Note that the `data` attributes this library adds are a feature of HTML5. If you
 
 ## Installation
 
-### NPM
+### npm
 
     npm install @rails/ujs --save
 

guides/source/5_1_release_notes.md

@@ -41,8 +41,8 @@ Major Features
 [Pull Request](https://github.com/rails/rails/pull/26836)
 
 Rails 5.1 allows managing JavaScript dependencies
-from NPM via Yarn. This will make it easy to use libraries like React, VueJS
-or any other library from NPM world. The Yarn support is integrated with
+from npm via Yarn. This will make it easy to use libraries like React, VueJS
+or any other library from npm world. The Yarn support is integrated with
 the asset pipeline so that all dependencies will work seamlessly with the
 Rails 5.1 app.