Fix variable name.

qipipe/staging/staging_helper.py

@@ -286,7 +286,7 @@ def next(self):
         # The visit subdirectory match pattern.
         vpat = self.collection.session_pattern
         logger(__name__).debug("The visit directory search pattern is %s..." %
-                               vpath)
+                               vpat)
 
         # The DICOM file search pattern depends on the scan type.
         dcm_pat_dict = self.collection.dcm_pat_dict