Update getting-started.rst

The ' will lead a Doctrine error, use " replace fix it

Author: efeencheung

tutorial/getting-started.rst

@@ -112,7 +112,7 @@ to reduce code duplication::
         protected $title;
 
         /**
-         * @PHPCR\Field(type='string', nullable=true)
+         * @PHPCR\Field(type="string", nullable=true)
          */
         protected $content;