Minor style fix

proposals/BVAL-496.adoc

@@ -49,13 +49,16 @@ These rules would make the following Threeten-Extra types supported:
 
 To make the current instant customizable, a new SPI which returns `java.time.Clock` to be used by the validation engine for validating `@Past` and `@Future` constraints should be added:
 
+[source,java]
+----
     package javax.validation.spi;
         
     import java.time.Clock;
         
     public interface ClockProvider {
         Clock getClock();
     }
+----
 
 BV implementations must use a default `ClockProvider` which always returns `java.time.Clock.systemDefault()`.