Fix video paths for external files

Sources/Generator/Content/Element.swift

@@ -65,7 +65,7 @@ struct Element {
     let sortIndex: Int?
 
     /**
-     All files which may occur in content but is stored externally.
+     All files which may occur in content but are stored externally.
 
      Missing files which would otherwise produce a warning are ignored when included here.
      - Note: This property defaults to an empty set.

Sources/Generator/Generators/PageContentGenerator.swift

@@ -215,6 +215,7 @@ struct PageContentGenerator {
         }
         // Also look in external files
         sources += selectVideoFiles(with: prefix, from: page.externalFiles)
+            .map { (page.relativePathToFileWithPath($0.url), $0.type) }
 
         // Require all video files
         sources.forEach {