Adjust default user name.

Index: samples/knot.sample.conf.in
--- samples/knot.sample.conf.in.orig
+++ samples/knot.sample.conf.in
@@ -3,7 +3,7 @@
 
 server:
     rundir: "@run_dir@"
-    user: knot:knot
+    user: _knot:_knot
     automatic-acl: on
 #    listen: [ 127.0.0.1@53, ::1@53 ]
 
