dcm2json: documentation issue blk-file-prefix

[malaterre]

Describe the bug

dcm2json should document its own options better.

To Reproduce

$ dcm2json -h

reports:

    --blk-file-suffix <suffix>   directory were files with extracted
                                 bulkdata are stored if the DICOM object
                                 is read from "standard input; if not
                                 specified, files are stored into the
                                 default temporary-file directory

Expected behavior

instead it should report:

    --blk-file-prefix <suffix>   suffix for generating file names for
                                 extracted bulkdata; 'tmp' by default