error: Fix "Converting to ERRP_GUARD()" doc on "valid at return"

Setting errp = NULL is wrong: the automatic error propagation still
propagates the dangling pointer _auto_errp_prop.local_err.  We need to
set *errp = NULL to clear the dangling pointer.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210125132635.1253219-1-armbru@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
diff --git a/include/qapi/error.h b/include/qapi/error.h
index eaa05c4..4a9260b 100644
--- a/include/qapi/error.h
+++ b/include/qapi/error.h
@@ -235,7 +235,7 @@
  *    error_propagate_prepend(errp, *errp, ...) by error_prepend(errp, ...)
  *
  * 4. Ensure @errp is valid at return: when you destroy *errp, set
- *    errp = NULL.
+ *    *errp = NULL.
  *
  * Example:
  *