fix doc of projection

Fix documentation of "[projection](http://docs.mongodb.org/manual/tutorial/project-fields-from-query-results)". Use "fields()"-method for inclusion or exclusion of fields from the result set. See https://groups.google.com/forum/#!searchin/mongodb-user/perl$20find/mongodb-user/FNNyRWqFtf8/GJ2gMcE-ErIJ