Update effects_overview.adoc

Add img-jmonkeyengine attribute to header.

src/docs/asciidoc/jme3/advanced/effects_overview.adoc

@@ -6,6 +6,7 @@
 :relfileprefix: ../../
 :imagesdir: ../..
 :uri-jmonkeyengine: https://github.com/jMonkeyEngine/jmonkeyengine/tree/master/
+:img-jmonkeyengine: https://github.com/jMonkeyEngine/jmonkeyengine/raw/master/
 :uri-forum: https://hub.jmonkeyengine.org/
 ifdef::env-github,env-browser[:outfilesuffix: .adoc]