Fix typo on ssh.ProxyCommand

On fabric 1.8.1 we got `AttributeError: 'module' object has no attribute 'proxycommand' error` by the typo.
```
fab test
[biiwork] Executing task 'test'
[biiwork] run: echo 'Done test.'
Traceback (most recent call last):
.../Python/fabric/fabric/network.py", line 99, in get_gateway
    sock = ssh.proxycommand(proxy_command)
AttributeError: 'module' object has no attribute 'proxycommand'
```
This patch fixed it:
```
fab test
[biiwork] Executing task 'test'
[biiwork] run: echo 'Done test.'
[biiwork] out: Done test.
```