Fix docstring for shelve_offload_instance in compute manager

The shelve_offload_instance method in the compute manager is using an
instance object now but the docstring for the instance param said it was
a dict.

Change-Id: I9376f09402ac00e570a58f9b58b2f4aa56a7e16d
Merge "Fix docstring for shelve_offload_instance in compute manager"