There's a typo in them commands to run sample in "Dynamic Events" in Flight Recorder API Programmer’s Guide:
https://docs.oracle.com/en/java/javase/14/jfapi/dynamic-events.html
java -XX:StartFlightRecording:filename=d.jfr DynamicSample.java
jfr print --events RandomString d.jfr
https://docs.oracle.com/en/java/javase/14/jfapi/dynamic-events.html
java -XX:StartFlightRecording:filename=d.jfr DynamicSample.java
jfr print --events RandomString d.jfr