Update mesh.adoc

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

@@ -113,7 +113,7 @@ a|setMode(Mesh.Mode.Points)
 a|Used to set mesh rendering modes, see below.
 
 a|getId()
-a|returns the Mesh ID
+a|returns the Mesh ID, default value is -1
 
 a|getTriangle(int,tri)
 a|returns data of triangle number `int` into variable `tri`