fix: add newline before opening code fence in html2text (#462)

From PR #462 by @jtanningbed

crawl4ai/html2text/__init__.py

@@ -1146,7 +1146,7 @@ class CustomHTML2Text(HTML2Text):
         # Handle pre tags
         if tag == "pre":
             if start:
-                self.o("```\n")  # Markdown code block start
+                self.o("\n```\n")  # Markdown code block start
                 self.inside_pre = True
             else:
                 self.o("\n```\n")  # Markdown code block end