Quickfix for copy and paste bug in futures.ipynb comments

**Describe the issue**:
In the [futures.ipynb](https://github.com/dask/dask-examples/blob/03cfcca533fae6647f7df0b1f6b350938168d0a0/futures.ipynb#L207)
there is an example with x, y, z variables, where in line 207 and 208 the comments for the assignments of y and z are indicating that the "inc" function is used, but instead "double" and "add" are used.

**Minimal Complete Verifiable Example**:
```ipynb
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"%%time\n",
"\n",
"for i in range(256):\n",
" x = client.submit(inc, i) # x = inc(i)\n",
" y = client.submit(double, x) # y = inc(x)\n",
" z = client.submit(add, x, y) # z = inc(y)\n",
" zs.append(z)"
]
}
```

should be

```ipynb
{
"cell_type": "code",
"execution_count": null,
"metadata": {},
"outputs": [],
"source": [
"%%time\n",
"\n",
"for i in range(256):\n",
" x = client.submit(inc, i) # x = inc(i)\n",
" y = client.submit(double, x) # y = double(x)\n",
" z = client.submit(add, x, y) # z = add(x, y)\n",
" zs.append(z)"
]
}
```

**Anything else we need to know?**:
no, this is really a quick fix

**Environment**:
[Line 207](https://github.com/dask/dask-examples/blob/03cfcca533fae6647f7df0b1f6b350938168d0a0/futures.ipynb#L207)
[Line 208](https://github.com/dask/dask-examples/blob/03cfcca533fae6647f7df0b1f6b350938168d0a0/futures.ipynb#L208)