Update editing.rst

I tried the column selection procedure as described in the documentation but was unable to see any results. So I experimented with it a lil and found that using my macbook pro trackpad I was able to make the changes I wanted, So I corrected the documentation. Please review and if approved apply it to the original documentation.

I made changes in section 'Column Selection' from line 63 to 65
The action you describe here maps to the following command and is not entirely correct:
```
    {
        "button": "button1", "modifiers": ["alt", "shift", "super"],
        "press_command": "drag_select",
        "press_args": {"by": "columns", "subtractive": true}
    },
```