Fix typo in exec.rb
Fix typo in exec.rb

"whitespace" not "whitespaces".

stanhu/haml,stanhu/haml,macressler/haml,tangposmarvin/haml,haml/haml,haml/haml,macressler/haml,sideci-sample/sideci-sample-haml,keithpitty/haml,tangposmarvin/haml,keithpitty/haml,haml/haml,dg1dd1ngs/haml,nazar-pc/haml,nazar-pc/haml,dg1dd1ngs/haml,haml/haml