Enhanced the example for the Singleton design pattern.

* 7d91e9c: Removed trailing whitespaces
* 78b8adc: Enhanced the example for the Singleton design pattern.
    - Moved static variable into the static creation method getInstance.
    - Replaced "new self();" with "new static" (late static binding to allow subclassing).
    - Changed visibility of __construct to protected.
    - Added the two private methods __clone and __wakeup.
    - Added DocBlock comments.
    - Updated description and enhanced with links to the PHP manual.