Update contributions.adoc

Changed Github repo description to be definite not seems.

src/docs/asciidoc/jme3/contributions.adoc

@@ -20,7 +20,7 @@ To install a jMonkeyEngine SDK plugin, go to Tools→Plugins→Available Plugins
 
 == Github Repo
 
-This seems to be the main repository for jmonkey contributions:
+This is the main repository for jmonkey contributions:
 link:https://github.com/jMonkeyEngine-Contributions[https://github.com/jMonkeyEngine-Contributions]