Update boken link

[Eaglesight02]

• There is a link in the "Sites related to your preferred programming language (For me C++)" section that cannot be reached:
  • what-is-garbage-collection : Demystify the garbage collection
• The link should actually point to an e-book on java's garbage collection.
• When trying to access the link:
  
• Gets redirected to Splunk's homepage
  
• The e-book was a resource used to be provided by the performance monitoring software company, 'Plumbr'.
• 'Plumbr' was acquired by 'Splunk' and hence, might explain the link being broken.
  
• The link must either be updated with a URL pointing to the correct e-book or must be removed completely.