Update worker.py (#12403)

Fix an error reproduced with python3.6.8, python3.5.2, python2.7.12:

$ echo "Hi!"| ./worker.py

Traceback (most recent call last):
  File "./worker.py", line 6, in <module>
    print("Processing " + sys.stdin.lines())
AttributeError: '_io.TextIOWrapper' object has no attribute 'lines'

content/en/examples/application/job/rabbitmq/worker.py

@@ -3,5 +3,5 @@
 # Just prints standard out and sleeps for 10 seconds.
 import sys
 import time
-print("Processing " + sys.stdin.lines())
+print("Processing " + sys.stdin.readlines()[0])
 time.sleep(10)