ocrd-tool.json: add directory content-type for model

sbb_binarize/ocrd-tool.json

@@ -19,6 +19,8 @@
         "model": {
           "description": "Directory containing HDF5 models. Can be an absolute path or a path relative to the current working directory or $SBB_BINARIZE_DATA environment variable (if set)",
           "type": "string",
+          "format": "uri",
+          "content-type": "text/directory",
           "required": true
         }
       }