doesn't escape % in @describeIn

This code:

``` R
#' @describeIn foo shortcut for foo
`%foo%` <- function(x, y) foo(x, y)
```

generates the following `foo.Rd`

``` latex
\name{foo}
\alias{\%foo\%}
\alias{foo}

 ...

\section{Functions}{
\itemize{
\item \code{%foo%}: shortcut for foo
}}
```

where the `%`s don't get escaped properly which breaks the LaTeX, as seen above.
