Fixing improperly closed file handle in ansible vault

The parentheses are left off the call, telling Python to return a method but not call the method.  You can see how this works in a REPL by calling ``close`` with and without the parentheses, noting how the file object changes.

```python
>>> f = open("/etc/passwd", "rb")
>>> f
<open file '/etc/passwd', mode 'rb' at 0x103ce26f0>
>>> f.close
<built-in method close of file object at 0x103ce26f0>
>>> f
<open file '/etc/passwd', mode 'rb' at 0x103ce26f0>
>>> f.close()
>>> f
<closed file '/Users/scott/.ansible/stow_vault_password', mode 'rb' at 0x103ce26f0>
```