ddns module: fix pylint warnings & docstring typo

```
************* Module salt.modules.ddns
W0621: 62,4:update: Redefining name 'update' from outer scope (line 31)
W0621: 88,4:delete: Redefining name 'update' from outer scope (line 31)
W0611:  9,0: Unused import salt
```