Typo fix in CSV export code & docs

[tiberio-baptista]

This quick PR fixes a typo present in Dicoogle's REST API pertaining to CSV exporting:

• The endpoint /export/cvs was changed to /export/csv;
• Related methods/code were refactored;
• The API docs were updated accordingly.

[Enet4]

We should not bother changing that endpoint. It is obsolete since Dicoogle version 2, so we are better off removing it entirely in future versions.

If I'm not mistaken, you are looking for POST /exportFile, available in dicoogle-client as issueExport, then GET /exportFile to download the CSV file.