README example compiles and its license summary matches LICENSE

## Summary

Fix three defects in `README.md`: a usage example that does not compile, a false statement of the license terms, and a broken reference link.

## Motivation

The example is the first thing a prospective user copies, and it fails at parse time. Separately, the license section tells readers they may use the package "without any conditions", which MIT does not grant.

Found in a documentation audit; the compile failures below are reproduced with `ghc -fno-code`.

## Scope

### The usage example does not compile — `README.md:23-31`

Three independent errors:

- `module main where` — parse error GHC-58481. Haskell module names must be capitalized.
- `generate` — GHC-88464, variable not in scope.
- `arbitrary` — GHC-88464, variable not in scope.

`Test.QuickCheck` is never imported. This version compiles against the current package environment:

```haskell
{-# LANGUAGE TypeApplications #-}

module Main where

import Network.URI (URI)
import Network.URI.Arbitrary ()
import Test.QuickCheck (arbitrary, generate)

main :: IO ()
main = generate (arbitrary @URI) >>= print
```

### The license summary is wrong — `README.md:18`

"You are free to use network-arbitrary without any conditions." `LICENSE:11` requires that the copyright notice and permission notice be included in copies. MIT is permissive, not condition-free.

### `[LICENSE]` is a broken reference link — `README.md:18`

The definitions at `README.md:51-56` are `arbitrary`, `hackage`, `issues`, `network-category`, `pull requests`, and `quickcheck` — there is no `license` entry, so the text renders literally as `[LICENSE]`. Line 37 uses an inline link and is unaffected.

## Acceptance criteria

- [ ] The README example compiles as written.
- [ ] The license section states MIT's attribution condition and links `LICENSE`.
- [ ] No undefined reference links remain in `README.md`.

## Additional context

Adjacent to #125 (Vale prose linting), which would not surface any of these: Vale skips fenced code blocks, does not resolve Markdown reference links, and cannot tell that a license summary misstates the license.

Start with README.md:18-31 and the reference definitions at README.md:51-56; compare the license wording with LICENSE:11 and run ghc -fno-code on the fenced Haskell example. Done means the example compiles, the MIT attribution condition and LICENSE link are correct, and no undefined reference links remain.

索引モデルが issue の本文から書いたものです。