[Minor] Fix key path in arc.conf examples (#1784)

conf/modules.d/arc.conf

@@ -16,7 +16,7 @@
 # https://rspamd.com/doc/modules/arc.html
 
 # To enable this module define the following attributes:
-# path = "/var/lib/rspamd/arc/$domain.$selector.key";
+# path = "${DBDIR}/arc/$domain.$selector.key";
 # OR
 # domain { ... }, if you use per-domain conf
 # OR
@@ -34,7 +34,7 @@ arc {
   # If false, messages from authenticated users are not selected for signing
   auth_only = true;
   # Default path to key, can include '$domain' and '$selector' variables
-  #path = "/var/lib/rspamd/arc/$domain.$selector.key";
+  #path = "${DBDIR}/arc/$domain.$selector.key";
   # Default selector to use
   selector = "arc";
   # If false, messages from local networks are not selected for signing
@@ -56,7 +56,7 @@ arc {
   #domain {
   #  example.com {
   #    # Private key path
-  #    path = "/var/lib/rspamd/arc/example.key";
+  #    path = "${DBDIR}/arc/example.key";
   #    # Selector
   #    selector = "ds";
   #  }