Update let in IR spec to reflect tests

[raymyers]

Updating IR readme to reflect let's 0 value in the test suite.

  {
    "name": "let",
    "source": {
      "0": "l",
      "l": "x",
      "v": {
        "0": "s",
        "v": "hi"
      },
      "t": {
        "0": "v",
        "l": "x"
      }
    },
    "cid": "baguqeeraijyrkjvaioixrfdxtpbuhq6h2oqfu33jtuioehjghlqye27uoziq"
  },

[raymyers]

Side note, the release node from the suite is also not in the doc.

  {
    "name": "release",
    "source": {
      "0": "@",
      "p": "welcome",
      "r": 1,
      "l": {
        "/": "baguqeeraqyvjt4lhfr66jw66yhg37xaj7tvdde5pifx7eqdq2a4zgzpffwaq"
      }
    },
    "cid": "baguqeeranj6ckbez2utdhyks57fmzj46qsfd3doc4vopsa323izbi3tmnsja"
  }