update documentation; sincedb_path = path+filename

fixes the following:

    :message=>"A plugin had an unrecoverable error. Will restart this plugin.\n  Plugin: <LogStash::Inputs::S3 [...] sincedb_path=>\"/opt/mypath\", region_endpoint=>\"us-east-1\">\n  Error: Is a directory - /opt/mypath", :level=>:error