Update unbound.md

[clwgh]

What does this PR aim to accomplish?:

Minor tweak to logfile section to ensure human-readable timestamps (default is Unix time)

How does this PR accomplish the above?:

Include an additional config line and updated description in the logfile section

Link documentation PRs if any are needed to support this PR:
n/a

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[DL6ER]

The option log-time-ascii is around since at least Unbound 1.4.21 (back in 2013) so no compatibility issues are expected.