docfix for vsphere_guest.py

Corrected parameter name from user to username in documentation

Author: IndyMichaelB

cloud/vmware/vsphere_guest.py

@@ -38,7 +38,7 @@
     description:
       - The virtual server name you wish to manage.
     required: true
-  user:
+  username:
     description:
       - Username to connect to vcenter as.
     required: true