Fixed: attributesstring to attributes

I have changed function names due to this error, 

#30

After changing the function name to `attributes` issue was solved

src/ChrisKonnertz/OpenGraph/OpenGraph.php

@@ -173,7 +173,7 @@ public function tag(string $name, $value, bool $prefixed = true) : self
      * @param bool     $prefixed   Add the "og"-prefix?
      * @return OpenGraph
      */
-    public function attributesstring (
+    public function attributes (
         string $tagName, 
         array $attributes = [], 
         array $valid = [],