update list.join example to work correctly

when I tried to perform the `list.join` docs example 2, I got error

```
list.join(l1,l2,.["name","age"])
 Error in .["name", "age"] : incorrect number of dimensions
```

I believe this should be instead as it is in this [`list.join` tests](https://github.com/renkun-ken/rlist/blob/master/tests/testthat/test-list.join.R#L13).

```
list.join(l1,l2,.[c("name","age")])
```

To test that this multi-key example is working I changed the data to have James `age = 22` to show that `age` and `name` must match for `list.join` to return.