Completed a full example of forked tests with grouping

The previous example of how to fork group of tests in different
JVMs was incomplete and not fully working.

a) The "testGrouping in Test" is needed, otherwise the grouping is
   not known to which phase to apply
b) Added a more complete Project definition to explain what the curly
   brackets section refers to and where the settings need to be included

The new complete example works out of the box :-)
Removed redundant semicolon
Making example specific to build.sbt

(see discussion on https://github.com/sbt/sbt/pull/1139)
Merge pull request #1139 from lucamilanesio/0.13

Completed a full example of forked tests with grouping