Doc: specializing 'write-value'

[slburson]

The documentation says to specialize write-value like this:

(defmethod jzon:write-value (writer (job job)) ...)

This is incorrect, because the existing method on (writer t) will be more specific than the new one on (t job), so will be called instead. The correct way is to specialize on both parameters:

(defmethod jzon:write-value ((writer jzon:writer) (job job)) ...)

[daninus14]

Might as well just make a quick PR on the docs

[daninus14]

Here's a PR to address this: #89

[Zulu-Inuoe]

Closed by #89