DOC: mailers: typo in 'hostname' description

fixed a typo caused by a copy/paste where <hostname> parameter
description was replaced by <emailaddr>...
diff --git a/doc/configuration.txt b/doc/configuration.txt
index 9a8b382..4fe410f 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -3066,7 +3066,7 @@
 
   Arguments :
 
-    <emailaddr> is the to email address to use when sending email alerts
+    <hostname> is the hostname to use when communicating with mailers
 
   By default the systems hostname is used.