chore: update custom timeout header name (#6)

Author: stainless-app[bot]

lib/finch-api/base_client.rb

@@ -246,8 +246,8 @@ def initialize(
       end
 
       timeout = opts.fetch(:timeout, @timeout).to_f.clamp((0..))
-      unless headers.key?("x-stainless-read-timeout") || timeout.zero?
-        headers["x-stainless-read-timeout"] = timeout.to_s
+      unless headers.key?("x-stainless-timeout") || timeout.zero?
+        headers["x-stainless-timeout"] = timeout.to_s
       end
 
       headers.reject! { |_, v| v.to_s.empty? }