Reformat release history so newest release is directly visible

The description text area in the Eclipse plugin install dialog is only
three lines tall by default, so this makes the newest entry visible
without scrolling.

org.eventb.texteditor.feature/feature.xml

@@ -8,8 +8,7 @@
 
    <description>
       A text editor for the Rodin platform to edit Event-B models
------------------------------------------------------------
-Release History:
+--- Release History ---
 3.5.0 - Compatibility with EventB-EMF 7.0.0 - fixes CamilleX dependency conflict. Fixed OOM error when top-level end keyword is missing.
 3.4.1 - Internal updates to the build process - no visible changes.
 3.4.0 - Release for Rodin 3.5 and related dependencies