add examples for country and country_revision in comment

Author: MarcelOliver

osi_trafficsign.proto

@@ -301,9 +301,11 @@ message TrafficSign
 
             // Specifies the country. See ISO 3166-1, alpha-2 codes
             // https://en.wikipedia.org/wiki/ISO_3166-1_alpha-2
+            // e.g. "Germany", "France", "Austria"
             optional string country = 8;
 
             // The year the traffic rules come into force
+            // e.g. "2017"
             optional string country_revision = 9;
 
             // Code identifier according to country code