Fix problem with Open3.popen3 where confirm_documentation task never finishes waiting for child processes

see: https://github.com/rubocop/rubocop-rspec/actions/runs/11360936578/job/31599655573

Author: ydah

Rakefile

@@ -72,10 +72,10 @@ end
 
 desc 'Confirm documentation is up to date'
 task confirm_documentation: :generate_cops_documentation do
-  _, _, _, process =
-    Open3.popen3('git diff --exit-code docs/')
+  _, _, status =
+    Open3.capture3('git diff --exit-code docs/')
 
-  unless process.value.success?
+  unless status.success?
     raise 'Please run `rake generate_cops_documentation` ' \
           'and add docs/ to the commit.'
   end