style: name unused binding in format_elixir_source case

Credo flags bare `_` as inconsistent with the project's `_foo` style.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: fahchen

lib/dprint_markdown_formatter.ex

@@ -262,7 +262,7 @@ defmodule DprintMarkdownFormatter do
 
       case formatted do
         "" -> {:ok, ""}
-        _ -> {:ok, formatted <> "\n"}
+        _non_empty -> {:ok, formatted <> "\n"}
       end
     end
   end