[#395] Fix escaping in haddock for Morley.Util.Interpolate

Description

This is a fixup following !989 (merged)

In the haddock comments for Morley.Util.Interpolate there are some inline code blocks (@) that contain a backslash, which cause some unintended escaping in the final result (see rendered in Hackage) whereas the goal was to get: \.

Related issue(s)

Minor fixup for #395 (closed)

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