travelynx.conf: add validation hint

examples/travelynx.conf

@@ -2,7 +2,7 @@
 # travelynx.conf must be a valid perl hash reference. String values must be
 # quoted and hash items must end with a comma. You can access environment
 # variables via $ENV, e.g. by writing $ENV{TRAVELYNX_DB_HOST} instead of
-# 'localhost'.
+# 'localhost'. You can validate via 'perl -c travelynx.conf'.
 
 {
 	# Base URL of this travelynx installation, e.g. "https://travelynx.de" for