Update materials_overview.adoc

Fixed broken link.

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

@@ -126,7 +126,7 @@ setFloat(“slopeTileFactor,1f);
 
 <a| Common/MatDefs/Misc/Particle.j3md       
 a| Used with texture masks for particle effects, or for point sprites. +
-The Quadratic value scales the particle for perspective view (link:http://code.google.com/p/jmonkeyengine/source/browse/trunk/engine/src/core/com/jme3/effect/ParticleEmitter.java[formula]). +
+The Quadratic value scales the particle for perspective view (link:https://github.com/jMonkeyEngine/jmonkeyengine/blob/master/jme3-core/src/main/java/com/jme3/effect/ParticleEmitter.java[formula]). +
 Does support an optional colored glow effect. +
 See also: <<jme3/beginner/hello_effects#,Hello Effects>> 
 a| setTexture(“Texture, assetManager.loadTexture(“)); +