Percentage sign '%' is not escaped in ... code block.

[twang2218]

Say we have a .R file with following documents:

#' ```R
#'
#' generate_eventlog() %>% create_pmap() %>% render_pmap_file("test.svg", format = "svg")
#' %
#' \%
#' ```

The generated .Rd will be:

\preformatted{
generate_eventlog() %>% create_pmap() %>% render_pmap_file("test.svg", format = "svg")
%
\\%
}

As % is not escaped, so everything after % will not be shown in the final document page.

generate_eventlog() 

\

I think this bug is similar to #640, maybe there are other places or characters are needed to be escaped.