machine: digital-ocean: Rename 'enable-ipv6' to 'enable-ipv6?'.

* gnu/machine/digital-ocean.scm (<digital-ocean-configuration>)[enable-ipv6]:
Rename to...
[enable-ipv6?]: ... this.
(deploy-digital-ocean): Adjust accordingly.
* doc/guix.texi (Invoking guix deploy): Adjust accordingly.