Update documentation.adoc

Fixed broken internal cross reference bookmark where the underscore (_) character changed to (-).

src/docs/asciidoc/documentation.adoc

@@ -65,15 +65,15 @@ a| Tutorials
 a| jMonkeyEngine SDK
 a| Other Documentation
 
-a| <<jme3#tutorials_for_beginners,jME3 beginner tutorials>>
+a| <<jme3#tutorials-for-beginners,jME3 beginner tutorials>>
 a| <<sdk#,jMonkeyEngine SDK Documentation and Video Tutorials>>
 a| link:http://javadoc.jmonkeyengine.org/[Full API JavaDoc]
 
-a| <<jme3#documentation_for_intermediate_users,jME3 intermediate articles>>
+a| <<jme3#documentation-for-intermediate-users,jME3 intermediate articles>>
 a| <<sdk/comic#,jMonkeyEngine SDK - the Comic :-)>>
 a| <<jme3/external/blender#,Blender Modeling Guide>>
 
-a| <<jme3#documentation_for_advanced_users,jME3 advanced documentation>>
+a| <<jme3#documentation-for-advanced-users,jME3 advanced documentation>>
 <a|
 a| <<jme3/faq#,Answers to Frequently Asked Questions>>