add Elixir School - OTP Concurrency to links.json (#1113)

Author: dongdigua

concepts/genserver/links.json

@@ -7,6 +7,10 @@
     "url": "https://hexdocs.pm/elixir/GenServer.html",
     "description": "Documentation - Genserver"
   },
+  {
+    "url": "https://elixirschool.com/en/lessons/advanced/otp_concurrency",
+    "description": "Elixir School - OTP Concurrency"
+  },
   {
     "url": "https://en.wikipedia.org/wiki/Actor_model",
     "description": "Actor model"