Fix missing type cast on getHeadingTextColour function

- A type cast had accidentally not been added to the getHeadingTextColour function, so has now been added

lib/Gocdb_Services/Config.php

@@ -390,7 +390,7 @@ public function getBackgroundColour3()
 
     public function getHeadingTextColour()
     {
-        $headingTextColour = $this->GetLocalInfoXML()->css->headingTextColour;
+        $headingTextColour = (string) $this->GetLocalInfoXML()->css->headingTextColour;
 
         return $headingTextColour;
     }