Update particle_emitters.adoc

Test to see if setting background color of cell works.

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

@@ -230,7 +230,7 @@ a| image:{img-jmonkeyengine}jme3-testdata/src/main/resources/Effects/Explosion/s
 
 <a| Effects/Smoke/Smoke.png
 a| 1*15
-a| image:{img-jmonkeyengine}jme3-testdata/src/main/resources/Effects/Smoke/Smoke.png[Smoke.png,width="96",height="32"]
+a|{set:cellbgcolor:black} image:{img-jmonkeyengine}jme3-testdata/src/main/resources/Effects/Smoke/Smoke.png[Smoke.png,width="96",height="32"]
 
 |===