Fix warn_if_in_single_mode incorrect message (#3111)

The message could either be in the positive `but Puma is configured to run in single mode (worker count = 0 )` or the negative `but Puma is not configured to run in cluster mode (worker count > 0 )`. I went with negative because it explicitly instructs what is required in order for hooks to run.

lib/puma/dsl.rb

@@ -1103,7 +1103,7 @@ def warn_if_in_single_mode(hook_name)
       if (@options[:workers] || 0) == 0
         log_string =
           "Warning: You specified code to run in a `#{hook_name}` block, " \
-          "but Puma is configured to run in cluster mode, " \
+          "but Puma is not configured to run in cluster mode (worker count > 0 ), " \
           "so your `#{hook_name}` block did not run"
 
         LogWriter.stdio.log(log_string)