Documentation edit for vsphere_guest module

I initially couldn't get the vpshere_guest module to work with a standard esxi host. After using pysphere to try to create a vm by hand, I realized that every standalone ESXi host has a default datacenter called ha-datacenter. This isn't displayed anywhere in the web-gui, and I couldn't find it in vmware's documentation. My initial attempt was leaving the datacenter as an empty string (I suspect this would be what others would try as well). I think it would be nice if the module documentation mentioned that standalone connections should use ha-datacenter. If I should make the change somewhere else in the docstring, let me know!