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/eval.c b/src/eval.c
index ea57cde..0579a85 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1112,7 +1112,7 @@
 	switch (om->ocm_access)
 	{
 	    case VIM_ACCESS_PRIVATE:
-		msg = e_cannot_access_private_variable_str;
+		msg = e_cannot_access_protected_variable_str;
 		break;
 	    case VIM_ACCESS_READ:
 		// If [idx] or .key following, read only OK.