Docx reader: Fix window path for image lookup.

Don't use os-sensitive "combine", since we always want the paths in our
zip-archive to use forward-slashes.

src/Text/Pandoc/Readers/Docx/Parse.hs

@@ -584,7 +584,7 @@ expandDrawingId s = do
   target <- asks (lookupRelationship s . envRelationships)
   case target of
     Just filepath -> do
-      bytes <- asks (lookup (combine "word" filepath) . envMedia)
+      bytes <- asks (lookup ("word/" ++ filepath) . envMedia)
       case bytes of
         Just bs -> return (filepath, bs)
         Nothing -> throwError DocxError