Cleanup %decoder

[suyashkumar]

At some point there seems to have been an inadvertent find and replace that snuck in to change %d to %decoder. Example

dicom/parse.go

Line 103 in 668a92a

panic(fmt.Sprintf("ReadElement failed to consume data: %decoder %decoder: %v", startLen, p.decoder.Len(), p.decoder.Error()))