rule-design: Update random ID generator (#1358)

* rule-design: Update random ID generator

Replace the hex generator with an alphanumeric number generator.

* Update pages/design/rule-design.md

Co-authored-by: Jean-Yves Moyen <jym@siteimprove.com>

Co-authored-by: Jean-Yves Moyen <jym@siteimprove.com>
Co-authored-by: Jey Nandakumar <jey.nandakumar@gmail.com>

pages/design/rule-design.md

@@ -28,7 +28,7 @@ For more details, see [ACT Rules Format: Structure](https://www.w3.org/TR/act-ru
 
 ### Rule identifier
 
-The 6-digit identifier is created using a random hex generator, such as https://www.browserling.com/tools/random-hex. The identifier remains the same if the rule name changes.
+The 6-character identifier is created using a random alpha-numeric generator, such as [random.org/strings](https://www.random.org/strings/?num=1&len=6&digits=on&loweralpha=on&format=html&rnd=new). The identifier remains the same if the rule name changes.
 
 ### Rule name