Fix typo in access configuration example

There's a typo in the access configuration example. Took me a while to find since I was opening with an editor without highlight.

Author: Rodrigo nonose

config/dev.exs.sample

@@ -1,7 +1,7 @@
 use Mix.Config
 
 config :gmail, :oauth2, [
- client_id: "CLIENT_ID
+ client_id: "CLIENT_ID",
  client_secret: "CLIENT_SECRET",
  access_token: "ACCESS_TOKEN",
  refresh_token: "REFRESH_TOKEN",