patch 8.1.0138: negative value of 'softtabstop' not used correctly

Problem:    Negative value of 'softtabstop' not used correctly.
Solution:   Use get_sts_value(). (Tom Ryder)
diff --git a/src/option.c b/src/option.c
index 50d42eb..504b92a 100644
--- a/src/option.c
+++ b/src/option.c
@@ -13016,7 +13016,7 @@
 
 /*
  * Return the effective softtabstop value for the current buffer, using the
- * 'tabstop' value when 'softtabstop' is negative.
+ * 'shiftwidth' value when 'softtabstop' is negative.
  */
     long
 get_sts_value(void)