Update jme3_source_structure.adoc

Turned note text into Admonition.

src/docs/asciidoc/jme3/jme3_source_structure.adoc

@@ -68,7 +68,10 @@ a| Native Bullet implementation pass:[C++] classes. +++<strike>jBullet</strike>+
 
 |===
 
-Note: Only one of the physics libraries can be used at a time as they replace each other.
+[NOTE]
+====
+Only one of the physics libraries can be used at a time as they replace each other.
+====
 
 
 === Plugins and Extra packages