Combine adjacent code spans into one.

> As with https://github.com/plt/racket/pull/296, @soegaard reported this issue after noticing it [here](https://github.com/soegaard/bind/blob/master/README.md)

These result from something like

    @racket[(x y)]

being treated by Scribble as multiple RktXXX items rather than one. As
a result the Markdown emitted was:

    `(``x`` ``y``)`

But obviously instead we want:

    `(x y)`

Kludgosity alert: Although it would probably be more-correct to
consolidate the RktXXX items at the Scribble structure level, I don't
easily see how. `@racket` is baking in the concept of Racket
lexing (classifying text as various kinds of Racket elements). This is
handy when the render will be HTML or Latek, and is benignly N/A when
it will be plain text. But it's a bit square-peg/round-hole when the
render will be Markdown. Rather than attempt to "un-lex" the Scribble
structures (I'm having trouble seeing how), I'm handling it
"after-the-fact" -- adjusting the generated Markdown text.

If anyone thinks the preceding is an elaborate rationalization for an
ugly kludge, I wouldn't argue, but I would need some help
understanding the preferable way to go about it.