Fix missing closing code tag in README (#288)

Adding missing tag above visitors section so it's not inside the codeblock.

Author: Hennell

README.md

@@ -181,6 +181,7 @@ $client->customers->search([
     "sort" => ["field" => "name", "order" => "ascending"],
     "pagination" => ["per_page" => 10]
 ]);
+```
 
 ## Visitors