Use better link for platform security/signing

The introduction in the Symbian SDK points to this document: file:///C:/Qt/4.6.3/doc/html/symbian-with-qt-introduction.html

Where it says
"To install your own applications on hardware, you need a signed .sis file. (Refer to the Platform Security Article for more informations about self-signing.)"
The link to Platform security is general Symbian C++ article. A more targeted set of articles for Qt developers are here:

http://developer.symbian.org/wiki/index.php/Qt_%26_Symbian_Platform_Security
http://developer.symbian.org/wiki/index.php/Qt_%26_Application_Signing
In particular the second article covers self signing