Include tserver.memory.maps.max in the example xml file

The default configuration for Accumulo is designed for a server. Being an Apache project, we will probably get a fair amount of users who just want to grab the code and spin it up quickly on a regular machine to try it out. I do not think we should change our default settings, since in the -env.sh file we have everything fairly well documented and java heap errors will be fairly explicit. However, the in memory map size setting can be more obscure, as it solely pops up in the .err file (if not using native map, which is not unreasonable for those coming straight off the shelf). So I think we should put this property in the site.xml-example file so that the setting name can be easily found and quickly adjusted without as much of a headache.