Update crowall.py

Modify a small error. It works on my mac.
![1](
7xrahm.com1.z0.glb.clouddn.com/屏幕快照_2016-11-05_14.13.18.png)

crowall.py

@@ -28,14 +28,14 @@ def crosswall(systemtype='Window'):
             os.system('pause')
             print 'It\'s done on Windows! And Try your browser!'
         elif systemtype == "Linux":
-            os.system('cp /etc/hosts /etc/hosts_bak')
+            os.system('cp /etc/hosts.txt /etc/hosts_bak')
             os.system('mv ./hosts /etc/hosts')
             os.system('pause')
             os.system('sudo /etc/init.d/networking restart ')
             print 'It\'s done on Linux! And Try your browser!'
         elif systemtype == "Darwin":
             os.system('sudo cp /etc/hosts /etc/host_bak')
-            os.system('sudo mv ./hosts /etc/hosts')
+            os.system('sudo mv ./hosts.txt /etc/hosts')
             os.system('sudo ifconfig en0 down && sudo ifconfig en0 up')
             print 'It\'s done on Mac! And Try your browser!'
     except Exception as e: