Fixes #1822 Update 02-libraries.md

If username is unrelated, then I propose `username` rather than `composer` or `acme` to avoid confusion.

doc/02-libraries.md

@@ -122,7 +122,7 @@ the `.gitignore`.
 Once you have a vcs repository (version control system, e.g. git) containing a
 `composer.json` file, your library is already composer-installable. In this
 example we will publish the `acme/hello-world` library on GitHub under
-`github.com/acme/hello-world`.
+`github.com/username/hello-world`.
 
 Now, To test installing the `acme/hello-world` package, we create a new
 project locally. We will call it `acme/blog`. This blog will depend on
@@ -150,7 +150,7 @@ We do this by adding a package repository specification to the blog's
         "repositories": [
             {
                 "type": "vcs",
-                "url": "https://github.com/acme/hello-world"
+                "url": "https://github.com/username/hello-world"
             }
         ],
         "require": {