DeveloperGuide: fix erroneous statement about Jackson
DeveloperGuide: fix erroneous statement about Jackson

Jackson is a JSON parsing library, not a XML parsing library.

Furthermore, the link http://wiki.fasterxml.com/JacksonHome seems to be
dead. Some googling reveals that the project likely moved to
https://github.com/FasterXML/jackson as their "home page".

While we are fixing up this section of the DeveloperGuide, lets convert
the list into a real one that doesn't use line breaks.

se-edu/addressbook-level3