Fix typo

(cherry picked from commit 3f94c3dcfc)

collects/meta/web/www/index.rkt

@@ -56,9 +56,9 @@
      @code{#lang web-server/insta
            ;; A "hello world" web server
            (define (start request)
-             (response/xexpr)
+             (response/xexpr
               '(html
-               (body "Hello World")))}
+                (body "Hello World"))))}
      @desc{This example implements a web server using the
        @elemcode{web-server/insta} language.  Each time a connection is made to
        the server, the @elemcode{start} function is called to get the HTML to