Update jme3_source_structure.adoc

Changed Data File Types table center column to literal because dot(.) prefixed lines in table cells are considered table titles and skipped.

src/docs/asciidoc/jme3/jme3_source_structure.adoc

@@ -239,19 +239,19 @@ a| File types
 a| purpose 
 
 a| /Common/MatDefs/*/ 
-a| .glsllib 
+l| .glsllib 
 a| Standard ShaderLibs 
 
 a| /Common/MatDefs/*/ 
-a| .j3md 
+l| .j3md 
 a| Standard Material Definitions 
 
 a| /Common/Materials/*/ 
-a| .j3m 
+l| .j3m 
 a| Standard Material 
 
 a| /Interface/Fonts/ 
-a| .fnt + .png 
+l| .fnt + .png 
 a| Standard Fonts 
 
 |===