update docstring

pyresample/bucket/__init__.py

@@ -210,7 +210,7 @@ def get_sum(self, data, fill_value=np.nan, skipna=True, empty_bucket_value=0):
         data : Numpy or Dask array
             Data to be binned and summed.
         fill_value : float
-            Fill value to mark missing/invalid values in the input data.
+            Fill value of the input data marking missing/invalid values.
             Default: np.nan
         skipna : boolean (optional)
             If True, skips missing values (as marked by NaN or `fill_value`) for the sum calculation