Update remove_artifacts.py to change string literals (probably used m…

…istakenly) to actual variables.

src/spikeinterface/preprocessing/remove_artifacts.py

@@ -166,7 +166,7 @@ def __init__(
                     assert l in artifacts.keys(), f"Artefacts are provided but label {l} has no value!"
             else:
                 assert (
-                    "ms_before" is not None and "ms_after" is not None
+                    ms_before is not None and ms_after is not None
                 ), f"ms_before/after should not be None for mode {mode}"
                 sorting = NumpySorting.from_times_labels(list_triggers, list_labels, recording.get_sampling_frequency())
                 sorting = sorting.save()