Fix typo in save/load ext feature enum name for improved breakdown.

src/saveload/extended_ver_sl.h

@@ -21,7 +21,7 @@
  */
 enum SlXvFeatureIndex {
 	XSLFI_NULL                          = 0,      ///< Unused value, to indicate that no extended feature test is in use
-	XLSFI_IMPROVED_BREAKDOWNS,                    ///< Improved breakdowns patch
+	XSLFI_IMPROVED_BREAKDOWNS,                    ///< Improved breakdowns patch
 
 	XSLFI_SIZE,                                   ///< Total count of features, including null feature
 };