Add missing leading slash for the put mapping endpoint (fixes #969)

Author: chrispappas

src/Elasticsearch/Endpoints/Indices/PutMapping.php

@@ -35,7 +35,7 @@ public function getURI(): string
             return "/_mappings/$type";
         }
         if (isset($index)) {
-            return "$index/_mapping";
+            return "/$index/_mapping";
         }
         throw new RuntimeException('Missing parameter for the endpoint indices.put_mapping');
     }