Javadoc and Formatting fixes. (#619)

Javadoc and Formatting fixes.

Also, extremely minor code changes which have been tested.
This commit is only part one of two commits that aim to fix problems with formatting in our project. In part two I will modify the Google Java Style Guide (since it closely matches our code style) for our project so there is guidance on how to format and document. 

* Updated PlotSquared URL
* Removed plugin acronyms
* Fixed a typo
* Fixed grammar
* Use modern block id's
* Update YouTube video URL

worldedit-core/src/main/java/com/sk89q/worldedit/event/extent/EditSessionEvent.java

@@ -89,7 +89,8 @@ public class EditSessionEvent extends Event implements Cancellable {
      *
      * @return the actor, which may be null if unavailable
      */
-    public @Nullable Actor getActor() {
+    @Nullable
+    public Actor getActor() {
         return actor;
     }
 
@@ -98,7 +99,8 @@ public class EditSessionEvent extends Event implements Cancellable {
      *
      * @return the world
      */
-    public @Nullable World getWorld() {
+    @Nullable
+    public World getWorld() {
         return world;
     }