Replace "nuimber" with "number" (#512)

Author: jakirkham

zarr/core.py

@@ -1507,7 +1507,7 @@ def _set_selection(self, indexer, value, fields=None):
         # necessary data from the value array and storing into the chunk array.
 
         # N.B., it is an important optimisation that we only visit chunks which overlap
-        # the selection. This minimises the nuimber of iterations in the main for loop.
+        # the selection. This minimises the number of iterations in the main for loop.
 
         # check fields are sensible
         check_fields(fields, self._dtype)