Fix an offence conflict to appease both cops

IfUnlessModifier insists on using the inline form, but it clearly knows
it won't fit its correction in LineLength's constraints and keeps
silent.

Its offence is only visible with the default 120 LineLength.

There is a number of offences like this, frankly I didn't look to close,
but this specific change appeals to me.

Rakefile

@@ -42,9 +42,12 @@ task confirm_config: :build_config do
   _, stdout, _, process =
     Open3.popen3('git diff --exit-code config/default.yml')
 
-  unless process.value.success?
-    raise "default.yml is out of sync:\n\n#{stdout.read}\nRun bin/build_config"
-  end
+  raise <<~ERROR unless process.value.success?
+    default.yml is out of sync:
+
+    #{stdout.read}
+    Run bin/build_config
+  ERROR
 end
 
 desc 'Confirm documentation is up to date'