Fix docstring in validator re regexp for frame number

movement/validators/files.py

@@ -289,7 +289,9 @@ def _file_contains_valid_frame_numbers(self, attribute, value):
           files.
 
         If the frame number is included as part of the image file name, then
-        it is expected as an integer led by at least one zero, followed by the
+        it is expected to be captured by the regular expression in the
+        `frame_regexp` attribute of the ValidVIATracksCSV object. The default
+        regexp matches an integer led by at least one zero, followed by the
         file extension.
 
         """