minor #35610 Fix Contracts autoloader typo (IonBazan)

This PR was merged into the 5.1-dev branch.

Discussion
----------

Fix Contracts autoloader typo

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | Fix #35526
| License       | MIT

Autoloader does not include new Deprecation component when installing `symfony/contracts` due to [typo](https://getcomposer.org/doc/04-schema.md#files).

Steps to reproduce:
```
composer dump-autoload -a
cat vendor/composer/autoload_files.php
```
Produces:
```
<?php

// autoload_files.php @generated by Composer

$vendorDir = dirname(dirname(__FILE__));
$baseDir = dirname($vendorDir);

return array(
    '0e6d7bf4a5811bfa5cf40c5ccd6fae6a' => $vendorDir . '/symfony/polyfill-mbstring/bootstrap.php',
    '25072dd6e2470089de65ae7bf11d3109' => $vendorDir . '/symfony/polyfill-php72/bootstrap.php',
    'f598d06aa772fa33d905e87be6398fb1' => $vendorDir . '/symfony/polyfill-intl-idn/bootstrap.php',
);
```

Commits
-------

d5bcaff00c Fix Contracts autoloader typo

src/Symfony/Contracts/composer.json

@@ -41,7 +41,7 @@
     },
     "autoload": {
         "psr-4": { "Symfony\\Contracts\\": "" },
-        "file": [ "Deprecation/deprecated.php" ],
+        "files": [ "Deprecation/deprecated.php" ],
         "exclude-from-classmap": [
             "**/Tests/"
         ]