lang handin-server/checker - improve docs for first timers

[maueroats]

Summary

The header line of the documentation for the Handin Server makes it sounds like there is a #lang handin-server/checkin. Installing the module makes no such header line permissible.

If the intention is that such a language line work, then fix and include it in an example. Otherwise edit documentation to make it clear how to use the module (example below).

Suggested improvements in documentation

Add a line to the first example in Section 6.3 Checker that shows #lang racket or
; intermediate student language if that is appropriate (I can't figure it out).

Edit example so that it actually works - remove ellipsis and put the appropriate provide (and require?) statements so that it exports something called checker. To offer something concrete:

#lang racket
(module the-checker ...   ; rest as written in docs
    (provide checker))
(require (submod "." the-checker))
(provide checker)

Steps to reproduce

• Racket version: 7.3
• Install package: handin
• Create an almost-empty file following the documentation for the Handin Server.

#lang handin-server/checker

Expected result

Empty program runs or gives error that the user has not written a checker: form.

Actual Result:

Error - language is not found.

standard-module-name-resolver: collection not found
  for module path: handin-server/checker/lang/reader
  collection: "handin-server/checker/lang"
  in collection directories: [...]