README: update suggested linting command

The `--show-deprecated` option was added to PHP Parallel Lint some years ago, but never documented, so I never knew it existed.

Enabling this option allows for deprecations which can be detected at compile time to be shown (in contrast to deprecations which can only be detected at run-time).

Refs:
* php-parallel-lint/PHP-Parallel-Lint#81
* JakubOnderka/PHP-Parallel-Lint#94

README.md

@@ -43,7 +43,7 @@ Typically, (a variation on) the following snippet would be added to the `compose
 ```json
     "scripts" : {
         "lint": [
-            "@php ./vendor/php-parallel-lint/php-parallel-lint/parallel-lint . -e php --exclude vendor --exclude .git"
+            "@php ./vendor/php-parallel-lint/php-parallel-lint/parallel-lint . -e php --show-deprecated --exclude vendor --exclude .git"
         ]
     }
 ```