fix the link to normalize_chunks

Author: keewis

doc/internals/how-to-add-new-backend.rst

@@ -455,5 +455,5 @@ In the first case Xarray uses the chunks size specified in
 ``preferred_chunks``.
 In the second case Xarray accommodates ideal chunk sizes, preserving if
 possible the "preferred_chunks". The ideal chunk size is computed using
-:py:func:`dask.core.normalize_chunks`, setting
+:py:func:`dask.array.core.normalize_chunks`, setting
 ``previous_chunks = preferred_chunks``.