Fix Cat/Fielddata endpoint URI

Accidentally had a reference to 'aliases' in the URI, which obviously
would make the api work incorrectly :s

src/Elasticsearch/Endpoints/Cat/Fielddata.php

@@ -50,7 +50,7 @@ protected function getURI()
         $uri   = "/_cat/fielddata";
 
         if (isset($fields) === true) {
-            $uri = "/_cat/aliases/$fields";
+            $uri = "/_cat/fielddata/$fields";
         }
 
         return $uri;