Fix typo serialization key in AbstractCinematicEvent (#2449)

The serialization key, for the class's field `initialDuration`, should has been `initialDuration` but
`initalDuration` was written.

Author: mondogo24

jme3-core/src/main/java/com/jme3/cinematic/events/AbstractCinematicEvent.java

@@ -292,7 +292,7 @@ public void write(JmeExporter ex) throws IOException {
         OutputCapsule oc = ex.getCapsule(this);
         oc.write(playState, "playState", PlayState.Stopped);
         oc.write(speed, "speed", 1);
-        oc.write(initialDuration, "initalDuration", 10);
+        oc.write(initialDuration, "initialDuration", 10);
         oc.write(loopMode, "loopMode", LoopMode.DontLoop);
     }
 
@@ -306,7 +306,15 @@ public void read(JmeImporter im) throws IOException {
         InputCapsule ic = im.getCapsule(this);
         playState = ic.readEnum("playState", PlayState.class, PlayState.Stopped);
         speed = ic.readFloat("speed", 1);
-        initialDuration = ic.readFloat("initalDuration", 10);
+        // Originally, "initialDuration" was serialized with the name "initalDuration".
+        // The next code ensure backward compatibility.
+        initialDuration = ic.readFloat("initialDuration", -1);
+        if (initialDuration < 0) {
+            initialDuration = ic.readFloat("initalDuration", -1);
+            if (initialDuration < 0) {
+                initialDuration = 10;
+            }
+        }
         loopMode = ic.readEnum("loopMode", LoopMode.class, LoopMode.DontLoop);
     }