Examples illustrating difference between legacy mapReduceTriplets usage and aggregateMessages usage has type issues on the reduce for both operators.

Being just an example-  changed example to reduce the message String by concatenation. Although non-optimal for performance.

Author: DEBORAH SIEGEL <deborahsiegel@DEBORAHs-MacBook-Pro.local>

Closes #4853 from d3borah/master and squashes the following commits:

db54173 [DEBORAH SIEGEL] fixed aggregateMessages example in graphX doc