[racket] Fix for dherman/json

Hey,

Here's a simple fix for the dherman/json:3:0 module defect #265 handling
negative numbers.

Replace line 173
  (let* ([sign (if (eq? (peek-char port) #\-) '(#\-) '())]
with
  (let* ([sign (if (eq? (peek-char port) #\-) (begin (read-char port)
'(#\-)) '())]
to consume the sign character.

Anybody managing the package these days? I fixed my local copy but it's
not a very persistent solution. Ideas?

-Markku