Merge pull request rurema#3293 from znz/queue-pop-example-unify

Thread::Queue#pop の nonblock 例を SizedQueue と統一

Author: znz

manual/api/thread/Queue.md

@@ -159,6 +159,7 @@ begin
   q.pop(true)
 rescue => e
   p e
+  p e.message
 end
 
 # => resource1