LaTeX writer: enclose backtick in {} in verbatim

We don't want ligatures like ` ?` ` (which produces `¿`) inside
`\texttt{}` environments, so we enclose the backtick in braces.

This fixes #3121

src/Text/Pandoc/Writers/LaTeX.hs

@@ -276,6 +276,7 @@ stringToLaTeX  ctx (x:xs) = do
        '€' -> "\\euro{}" ++ rest
        '{' -> "\\{" ++ rest
        '}' -> "\\}" ++ rest
+       '`' | ctx == CodeString -> "{`}" ++ rest
        '$' | not isUrl -> "\\$" ++ rest
        '%' -> "\\%" ++ rest
        '&' -> "\\&" ++ rest