chore: use correct arguments for custom algo example

Author: aedryan

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 **Fixes and enhancements:**
 
+- Updated README to correctly document `OpenSSL::HMAC` documentation [#617](https://github.com/jwt/ruby-jwt/pull/617) ([@aedryan](https://github.com/aedryan))
 - Your contribution here
 
 ## [v2.9.1](https://github.com/jwt/ruby-jwt/tree/v2.9.1) (2024-09-23)

README.md

@@ -240,7 +240,7 @@ module CustomHS512Algorithm
   end
 
   def self.sign(data:, signing_key:)
-    OpenSSL::HMAC.digest(OpenSSL::Digest.new('sha512'), data, signing_key)
+    OpenSSL::HMAC.digest(OpenSSL::Digest.new('sha512'), signing_key, data)
   end
 
   def self.verify(data:, signature:, verification_key:)

spec/integration/readme_examples_spec.rb

@@ -458,7 +458,7 @@ def self.alg
         end
 
         def self.sign(data:, signing_key:)
-          OpenSSL::HMAC.digest(OpenSSL::Digest.new('sha512'), data, signing_key)
+          OpenSSL::HMAC.digest(OpenSSL::Digest.new('sha512'), signing_key, data)
         end
 
         def self.verify(data:, signature:, verification_key:)