Closing backticks for Violation example (#375)

Author: jrinehart-buf

proto/protovalidate/buf/validate/validate.proto

@@ -4837,6 +4837,7 @@ message Violations {
 //         expression: "this < 18 ? 'User must be at least 18 years old' : ''",
 //     }];
 // }
+// ```
 //
 // It could produce the following violation:
 //