fixed typo in plugin

src/metadata_transfer.cpp

@@ -95,7 +95,6 @@ namespace libtorrent { namespace
 		return ret;
 	}
 
-
 	struct metadata_plugin : torrent_plugin
 	{
 		metadata_plugin(torrent& t)
@@ -217,7 +216,7 @@ namespace libtorrent { namespace
 			m_metadata_size = total_size;
 		}
 
-		void piece_pass(int)
+		void on_piece_pass(int)
 		{
 			// if we became a seed, copy the metadata from
 			// the torrent before it is deallocated