[Minor] Fix configuration example

conf/modules.d/greylist.conf

@@ -24,7 +24,7 @@ greylist {
   key_prefix = "rg"; # default hash name
   max_data_len = 10k; # default data limit to hash
   message = "Try again later"; # default greylisted message
-  #symbol = 'GREYLIST'; # Symbol to insert
+  #symbol = "GREYLIST"; # Symbol to insert
   action = "soft reject"; # default greylisted action
   ipv4_mask = 19; # Mask bits for ipv4
   ipv6_mask = 64; # Mask bits for ipv6