Update src/parcels/_core/fieldset.py

Co-authored-by: Erik van Sebille <e.vansebille@uu.nl>

Author: VeckoTheGecko

src/parcels/_core/fieldset.py

@@ -226,7 +226,7 @@ def from_ugrid_conventions(
         ----------
         ds : uxarray.UxDataset
             uxarray.UxDataset as obtained from the uxarray package but with appropriate named vertical dimensions
-        vector_fields : Mapping[str, tuple[str, ...]] or None, optional
+        vector_fields : Mapping[str, tuple[str, ...]], optional
             Mapping of vector field names to tuples of component variable names in the dataset.
             For example, ``{"UV": ("U", "V"), "UVW": ("U", "V", "W")}``.
             If omitted (default), vector fields are auto-discovered from standard variable names (``U``/``V``/``W``).