simplify example concurrent_download.py

- do not use Pool as it's already demoed in dns_mass_resolve.py

examples/concurrent_download.py

@@ -4,24 +4,18 @@
 
 urls = ['http://www.google.com', 'http://www.yandex.ru', 'http://www.python.org']
 
+import gevent
 from gevent import monkey
 monkey.patch_socket() # patches regular socket to yield to other greenlets
 
 import urllib2
-from gevent.pool import Pool
-
-# Pool accepts one optional argument - maximum number of concurrent
-# coroutines that can be active at any given moment.
-# Try passing 1 or 2 here to see the effect.
-pool = Pool()
 
 def print_head(url):
     print 'Starting %s' % url
     data = urllib2.urlopen(url).read()
     print '%s: %s bytes: %r' % (url, len(data), data[:50])
 
-for url in urls:
-    pool.spawn(print_head, url)
+jobs = [gevent.spawn(print_head, url) for url in urls]
 
-pool.join()
+gevent.joinall(jobs)