Update docs to reflect keys being prepended now

[mhkeller]

From the changes made in https://github.com/pbeshai/tidy/issues/34, keys are now prepended on objects in groupBy but the docs still show them being appended – groupBy.entries() for example:

const data = [
  { str: 'a', ing: 'x', foo: 'G', value: 1 },
  { str: 'b', ing: 'x', foo: 'H', value: 100 },
  { str: 'b', ing: 'x', foo: 'K', value: 200 },
  { str: 'a', ing: 'y', foo: 'G', value: 2 },
  { str: 'a', ing: 'y', foo: 'H', value: 3 },
  { str: 'a', ing: 'y', foo: 'K', value: 4 },
  { str: 'b', ing: 'y', foo: 'G', value: 300 },
  { str: 'b', ing: 'z', foo: 'H', value: 400 },
  { str: 'a', ing: 'z', foo: 'K', value: 5 },
  { str: 'a', ing: 'z', foo: 'G', value: 6 },
]

tidy(
  data,
  groupBy('str', [
    summarize({ total: sum('value') })
  ], groupBy.entries())
)
// output:
[
  ["a", [{"total": 21, "str": "a"}]], 
  ["b", [{"total": 1000, "str": "b"}]]
]

But putting that into the playground gives me:

[
  ["a", [{"str": "a", "total": 21}]], 
  ["b", [{"str": "b", "total": 1000}]]
]

[pbeshai]

Thanks for the heads-up! PRs welcome