Update Rakefile

Co-authored-by: Benjamin Quorning <[email protected]>

Rakefile

@@ -72,7 +72,7 @@ end
 
 desc 'Confirm documentation is up to date'
 task confirm_documentation: :generate_cops_documentation do
-  stdin, _, process =
+  stdout, _stderr, status =
     Open3.capture3('git diff --exit-code docs/')
 
   unless process.to_i.zero?