manual: fix meta.description in ruby example expression

Noted by @namore on github

doc/languages-frameworks/ruby.xml

@@ -30,7 +30,7 @@ bundlerEnv {
 
   meta = with lib; {
     description = "A monitoring framework that aims to be simple, malleable,
-and scalable.";
+and scalable";
     homepage    = http://sensuapp.org/;
     license     = with licenses; mit;
     maintainers = with maintainers; [ theuni ];