fix: PlayerEnterPlotEvent (java doc) description

[RedstoneFuture]

Overview

In this small Pull-Request the description of the PlayerEnterPlotEvent is fixed.

Description

• The event should be triggered when the player "enters" the plot. And not when he "leaves" it.
• The wording now corresponds to the description of the PlayerLeavePlotEvent event.

Submitter Checklist

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1. Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
2. Ensure that the pull request title represents the desired changelog entry.
3. New public fields and methods are annotated with @since TODO.
4. I read and followed the contribution guidelines.