Fix up unescaped tilde; reported by several people.
diff --git a/Doc/ref/ref5.tex b/Doc/ref/ref5.tex
index 8101cb9..fb8e3dd 100644
--- a/Doc/ref/ref5.tex
+++ b/Doc/ref/ref5.tex
@@ -629,7 +629,7 @@
 \begin{productionlist}
   \production{u_expr}
              {\token{power} | "-" \token{u_expr}
-              | "+" \token{u_expr} | "~" \token{u_expr}}
+              | "+" \token{u_expr} | "{\~}" \token{u_expr}}
 \end{productionlist}
 
 The unary \code{-} (minus) operator yields the negation of its