Update the "debugging" sample code in README.md (#1724)

Update README.md

Update the sample code for enabling HTTP debugging in Faraday

Author: areitz

README.md

@@ -597,7 +597,7 @@ traffic:
 
 ```ruby
 stack = Faraday::RackBuilder.new do |builder|
-  builder.use Faraday::Retry::Middleware, exceptions: Faraday::Request::Retry::DEFAULT_EXCEPTIONS + [Octokit::ServerError] # or Faraday::Request::Retry for Faraday < 2.0
+  builder.use Faraday::Retry::Middleware, exceptions: Faraday::Retry::Middleware::DEFAULT_EXCEPTIONS + [Octokit::ServerError] # or Faraday::Request::Retry for Faraday < 2.0
   builder.use Octokit::Middleware::FollowRedirects
   builder.use Octokit::Response::RaiseError
   builder.use Octokit::Response::FeedParser