rabbitmq.conf.example: this should be a comment line

(cherry picked from commit 5d1ae73)

Author: michaelklishin

docs/rabbitmq.conf.example

@@ -579,7 +579,7 @@
 # management.ssl.ciphers.9 = DHE-RSA-AES256-GCM-SHA384
 
 ## URL path prefix for HTTP API and management UI
-management.path_prefix = /a-prefix
+# management.path_prefix = /a-prefix
 
 ## One of 'basic', 'detailed' or 'none'. See
 ## https://rabbitmq.com/management.html#fine-stats for more details.