patch 8.2.1356: Vim9: cannot get the percent register

Problem:    Vim9: cannot get the percent register.
Solution:   Check for readable registers instead of writable. (closes #6566)
diff --git a/src/vim9compile.c b/src/vim9compile.c
index 5999c39..cfd4ff1 100644
--- a/src/vim9compile.c
+++ b/src/vim9compile.c
@@ -3565,7 +3565,7 @@
 	semsg(_(e_syntax_at), *arg - 1);
 	return FAIL;
     }
-    if (!valid_yank_reg(**arg, TRUE))
+    if (!valid_yank_reg(**arg, FALSE))
     {
 	emsg_invreg(**arg);
 	return FAIL;