fix documentation of unmounting volumes

little fix of the doc:
>> server.unmount(ubuntu_image_uuid)
NoMethodError: undefined method `unmount' for #<Fog::Compute::CloudSigma::Server:0x00000004739f48>
        from (irb):84:in `<top (required)>'
        from /home/toni/.gem/ruby/2.1.0/gems/fog-1.21.0/bin/fog:76:in `block in <top (required)>'
        from /home/toni/.gem/ruby/2.1.0/gems/fog-1.21.0/bin/fog:76:in `catch'
        from /home/toni/.gem/ruby/2.1.0/gems/fog-1.21.0/bin/fog:76:in `<top (required)>'
        from /home/toni/.gem/ruby/2.1.0/bin/fog:23:in `load'
        from /home/toni/.gem/ruby/2.1.0/bin/fog:23:in `<main>'
>> server.unmount_volume(ubuntu_image_uuid)
[] (works)