Fix typo: str.startwith() --> str.startswith()

While this is necessary, there's still something funky going on.

```python
from plumbum import SshMachine
remote_box = SshMachine('10.10.10.10')
p = remote_box.path('~')
p
>> <RemotePath /Users/user/(0, u'/Users/user\n', u'')>
print p
>> /Users/user/(0, u'/Users/user\n', u'')
```