[GraphX] Modify option name according to example doc in SynthBenchmark

Now graphx.SynthBenchmark example has an option of iteration number named as "niter". However, in its document, it is named as "niters". The mismatch between the implementation and document causes certain IllegalArgumentException while trying that example.

Author: Grace <jie.huang@intel.com>

Closes #2888 from GraceH/synthbenchmark and squashes the following commits:

f101ee1 [Grace] Modify option name according to example doc