Fix message for the assertion (#12)

The message of this exception must contain the value if the `$index`, not the `$value`.

Author: sitya

src/Auth/Process/EntityCategory.php

@@ -104,7 +104,7 @@ public function __construct(array $config, $reserved)
 
             Assert::string(
                 $index,
-                "Unspecified allowed attributes for the '$value' category.",
+                "Identifier of a category must be a string. '$index' is set.",
                 Error\ConfigurationError::class,
             );