Fixed deprecation warning on category test

Signed-off-by: snipe <snipe@snipe.net>

tests/Unit/CategoryTest.php

@@ -20,7 +20,7 @@ class CategoryTest extends TestCase
          ];
         $errors = $a->getErrors();
         foreach ($fields as $field => $fieldTitle) {
-            $this->assertEquals($errors->get($field)[0], "The ${fieldTitle} field is required.");
+            $this->assertEquals($errors->get($field)[0], "The $fieldTitle field is required.");
         }
     }