Example reordering columns in table docs broken

The following example in `modify_table.rst` (which runs in the context of all of the code above it):

```
  >>> t_acb = t['a','c','b']
  >>> neworder = ('a','c','b')
  >>> t_acb = t[neworder]
```

gives the following exception:

`ValueError: Illegal type <type 'tuple'> for table item access`

```
Traceback (most recent call last):

  File "/usr/lib64/python2.7/doctest.py", line 1289, in __run
    compileflags, 1) in test.globs

  File "<doctest modify_table.rst[35]>", line 1, in <module>

  File "astropy/table/table.py", line 1571, in __getitem__
    .format(type(item)))

ValueError: Illegal type <type 'tuple'> for table item access
```