utils/github.rb: use parallel assignments rather than indices

Co-Authored-By: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/utils/github.rb

@@ -341,8 +341,8 @@ module GitHub
   end
 
   def check_fork_exists(repo)
-    username = api_credentials[1]
-    reponame = repo.split("/")[1]
+    _, username = api_credentials
+    _, reponame = repo.split("/")
     json = open_api(url_to("repos", username, reponame))
     return false if json["message"] == "Not Found"