Use thin vert for single char errors

diff --git a/ast b/ast
index 5d3af4e1897a1da1c9e5eb1ba26711cb46e75829..88118d803a3256e2443b864a8bb3ab8c154289ba 160000 (submodule)
--- a/ast
+++ b/ast
@@ -1 +1 @@
-Subproject commit 5d3af4e1897a1da1c9e5eb1ba26711cb46e75829
+Subproject commit 88118d803a3256e2443b864a8bb3ab8c154289ba