Fixed varname for endpoint_region

Var were  misnamed, therefore the plugin was failing on : 

Exception in thread "LogStash::Runner" org.jruby.exceptions.RaiseException: (TypeError) can't convert nil into String
	at org.jruby.RubyString.+(org/jruby/RubyString.java:1137)
	at RUBY.aws_s3_config(file:/opt/logstash/server/lib/logstash-1.2.2.dev-flatjar.jar!/logstash/outputs/s3.rb:153)
	at RUBY.write_on_bucket(file:/opt/logstash/server/lib/logstash-1.2.2.dev-flatjar.jar!/logstash/outputs/s3.rb:185)
	at RUBY.upFile(file:/opt/logstash/server/lib/logstash-1.2.2.dev-flatjar.jar!/logstash/outputs/s3.rb:221)
	at org.jruby.RubyArray.each(org/jruby/RubyArray.java:1617)
	at RUBY.upFile(file:/opt/logstash/server/lib/logstash-1.2.2.dev-flatjar.jar!/logstash/outputs/s3.rb:213)
	at RUBY.receive(file:/opt/logstash/server/lib/logstash-1.2.2.dev-flatjar.jar!/logstash/outputs/s3.rb:327)
	at RUBY.worker_setup(file:/opt/logstash/server/lib/logstash-1.2.2.dev-flatjar.jar!/logstash/outputs/base.rb:65)