[README] Update DriveV3 example to use correct attributes

[garyhtou]

This DriveV3 example attempts to call outdated V2 attributes that no longer exist in V3.

• Google::Apis::DriveV3::FileList#items no longer exists in V3; it's been renamed to files.
• Google::Apis::DriveV3::File#title no longer exists in V3; it's been renamed to name.

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