patch 8.2.4620: two letter substitute commands don't work

Problem:    Two letter substitute commands don't work. (Yegappan Lakshmanan)
Solution:   Invert condition.
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 7c2fe3d..65f4221 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -3431,7 +3431,7 @@
     static int
 one_letter_cmd(char_u *p, cmdidx_T *idx)
 {
-    if (!in_vim9script())
+    if (in_vim9script())
 	return FALSE;
     if (*p == 'k')
     {