Update index.md (#254)

Modify import line in order to fix:
```
NameError: name 'maas' is not defined
```

Author: dmascialino

doc/client/index.md

@@ -11,7 +11,7 @@ Web API.
 ```python
 #!/usr/bin/env python3.6
 
-from maas.client import connect
+import maas.client
 
 # Replace … with an API key previously obtained by hand from
 # http://$host:$port/MAAS/account/prefs/.