doc: remove redundant explanation of format

Simplify format explanation for urlObject.auth.

PR-URL: #22324
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>

doc/api/url.md

@@ -921,8 +921,8 @@ The legacy `urlObject` (`require('url').Url`) is created and returned by the
 The `auth` property is the username and password portion of the URL, also
 referred to as _userinfo_. This string subset follows the `protocol` and
 double slashes (if present) and precedes the `host` component, delimited by an
-ASCII "at sign" (`@`). The format of the string is `{username}[:{password}]`,
-with the `[:{password}]` portion being optional.
+ASCII "at sign" (`@`). The string is either the username, or it is the username
+and password separated by `:`.
 
 For example: `'user:pass'`.