message: typo in docs: one ==> once
diff: @@ -120,7 +120,7 @@ dataset3 = dataset3.filter(lambda x, (y, z): ...)
  ### Creating an iterator
  -One you have built a `Dataset` to represent your input data, the next step is to
  +Once you have built a `Dataset` to represent your input data, the next step is to
  create an `Iterator` to access elements from that dataset. The `Dataset` API
  currently supports the following iterators, in increasing level of
  sophistication:
  