[Rd] R-exts.texi: suggestion for small change to Vignette section (PR#7323)

[S.J.Eglen at damtp.cam.ac.uk]

I recently wrote a vignette, with the following at the top of the .Rnw
file:

%%\VignetteIndexEntry{How to use look up tables for h() functions}
%%\VignetteDepends{sjedmin, spatstat}

Using vExplorer() on this function, I got an error from this part of
getVigInfo():

    lines <- grep("^%[[:space:]]*\\\\Vignette", file)
    if (length(lines) == 0) 
        stop("File ", vig, " does not appear to be a vignette file, ", 
            "no vignette metadata available.")

The error occurs here because the grep expects only one % at the start
of the line, whereas I had %%.  (I often use two at the start of a
line as I'm used to the ESS/Emacs coding habits of putting ## at the
start of a line.)  Changing %% to % solved the problem okay.  

So, my minor suggestion to R-exts.texi, at line 1011:

@code{\VignetteIndexEntry} statement is best placed in La at TeX{} comment,

is that maybe it should say either:

"in a La at TeX{} comment"

or 

"in a La at TeX{} comment with a single %"

although admittedly that sounds a bit clunky!

Thanks, Stephen