Fix Saga article link from `ProcessManager` Javadoc

[alexander-yevsyukov]

The ProcessManager class has a link to the article titled “Clarifying the Saga pattern“. The URL is this:

http://kellabyte.com/2012/05/30/clarifying-the-saga-pattern/

The site at this location gives the error:

Your PHP installation appears to be missing the MySQL extension which is required by WordPress.

There's a cached version of the article at archive.org.

We need to fix the link in our documentation so that users are not faced with the PHP error. Options are:

• contact the maintainer of kellabyte.com with the request to fix the PHP problem;
• update the link to the archived version;
• find a similar article and update the link.