docs: Fix "beginning with accounts" terminology in the README

Prior to this, the very first example of how to use this module used the
wrong terminology and had an incorrect explanation of what the example
was doing.

1) `accounts::user` is a defined type; it's not a class.
2) The users being created are not `Dan` and `Morgan`; they are `dan`
   and `morgan`
3) I tried to clarify that the `accounts::user` define doesn't have to
   be used in a Puppet-managed node's manifest, just that the example
   being shown chose to put it there.

README.md

@@ -25,7 +25,7 @@ This module works on many UNIX/Linux operating systems. It does not support conf
 
 ### Beginning with accounts
 
-Declare the `accounts` class in a Puppet-managed node's manifest:
+Declare the `accounts::user` defined type for each local user you want to manage:
 
 ~~~puppet
 node default {
@@ -34,7 +34,7 @@ node default {
 }
 ~~~
 
-The above example creates accounts, home directories, and groups for Dan and Morgan.
+The above example creates accounts, home directories, and groups for `dan` and `morgan` in the default node group.
 
 ## Usage