storage: fix reference to CopyFrom in doc comment
storage: fix reference to CopyFrom in doc comment

Change-Id: I40e19e1b28ff640a34c65e5c35fdd6c97ed36866
Reviewed-on: https://code-review.googlesource.com/14410
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go