fix: Fix metadata code comment

It was copy-pasted from SwiftMarkdown and doesn't make sense here

Author: kevinrenskers

Sources/Parsley/Document.swift

@@ -13,8 +13,6 @@ public struct Document {
   public let body: String
 
   /// Any metadata values found at the top of the Markdown document.
-  /// See https://python-markdown.github.io/extensions/meta_data/ for more information.
-  /// You'll need to use the `meta` extension for this to work.
   public let metadata: [String : String]
 
   internal init(title: String?, rawBody: String, body: String, metadata: [String : String]) {