fix a typo

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38405 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ChangeLog

@@ -64,7 +64,7 @@ Sat Dec 15 08:05:56 2012  KOSAKI Motohiro  <kosaki.motohiro@gmail.com>
 	test_main_thread_status_at_exit, test_thread_status_in_trap,
 	test_thread_status_raise_after_kill, test_mutex_owned,
 	test_mutex_owned2): move these tests from TestThreadGroup class
-	  to TestThread becuase they are not thread group tests.
+	  to TestThread because they are not thread group tests.
 
 	* test/ruby/test_thread.rb (test_thread_status_raise_after_kill):
 	  add t.join.