patch 9.0.2096: Vim9: confusing usage of private

Problem:  Vim9: confusing usage of private
Solution: clarify and use protected keyword instead

[vim9class] document `_` as protected instead of private

fixes #13504
closes: #13520

Signed-off-by: Ernie Rael <errael@raelity.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/src/vim9execute.c b/src/vim9execute.c
index ff54bcb..997dfa0 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -2248,7 +2248,7 @@
 	    {
 		if (*member == '_')
 		{
-		    emsg_var_cl_define(e_cannot_access_private_variable_str,
+		    emsg_var_cl_define(e_cannot_access_protected_variable_str,
 							m->ocm_name, 0, cl);
 		    status = FAIL;
 		}