Amend README testing steps

Vectorial1024 · Vectorial1024 · commit 4799949c6d09 · 2024-10-02T22:41:11.000+08:00
diff --git a/README.md b/README.md
@@ -67,7 +67,7 @@ $validObject->code = "something else"; // PHP runtime error: $code is read-only
 via PHPUnit; first ensure PHPUnit has been set up correctly:
 
 ```sh
-composer install
+composer update
 ```
 
 Then:
