Fix up unescaped tilde; reported by several people.

Doc/ref/ref5.tex

@@ -629,7 +629,7 @@ All unary arithmetic (and bit-wise) operations have the same priority:
 \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