Fixes duration unit for days from 'f' to 'd'

see:

```
java -XX:+FlightRecorder -XX:+FlightRecorder -XX:StartFlightRecording=maxsize=10000,maxage=10000m,delay=5f,filename=flight-native.jfr -Xlog:jfr  Main
java.lang.IllegalArgumentException: Integer parsing error nanotime value: illegal unit

whereas

java -XX:+FlightRecorder -XX:+FlightRecorder -XX:StartFlightRecording=maxsize=10000,maxage=10000m,delay=5d,filename=flight-native.jfr -Xlog:jfr  Main
[0.562s][info][jfr] Flight Recorder initialized
[0.562s][info][jfr] Created repository /tmp/2021_08_03_01_16_07_454665
[0.567s][info][jfr] Scheduled recording "1" (1) to start at 2021-08-08T01:16:07.684825
```

Signed-off-by: Michal Karm Babacek <karm@redhat.com>

substratevm/src/com.oracle.svm.jfr/src/com/oracle/svm/jfr/JfrManager.java

@@ -193,7 +193,7 @@ private static Long parseDuration(Map<JfrStartArgument, String> args, JfrStartAr
                     return Duration.ofMinutes(time).toNanos();
                 } else if ("h".equals(unit)) {
                     return Duration.ofHours(time).toNanos();
-                } else if ("f".equals(unit)) {
+                } else if ("d".equals(unit)) {
                     return Duration.ofDays(time).toNanos();
                 }
                 throw new IllegalArgumentException("Unit is invalid.");
@@ -233,6 +233,7 @@ private static Long parseMaxSize(Map<JfrStartArgument, String> args, JfrStartArg
                     case 'G':
                         return number * 1024 * 1024 * 1024;
                     default:
+                        // Unknown unit, number is treated as plain bytes
                         return number;
                 }
             } catch (IllegalArgumentException e) {