improve type definition of write-plist

[joergen7]

The function write-plist has been typed

(-> Plist-Dict Output-Port Void)

However, the untyped version has a less restrictive contract on the first argument:

(plist-value? output-port? . -> . void?)

This pr relaxes the type of the first argument to Plist-Value, thus being

(-> Plist-Value Output-Port Void)

[github-actions]

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