doc: include gb-ns2.adoc, not gb-variants.adoc

Adjust to merge of gb-variants.adoc into gb-ns2.adoc in
osmo-gsm-manuals I902b850528cbc04bd469590babd84cccf64300e8.

Change-Id: I17b182db6068ca583078c66c9576029a28016fd4
diff --git a/doc/manuals/osmogbproxy-usermanual.adoc b/doc/manuals/osmogbproxy-usermanual.adoc
index 06bc098..454cf69 100644
--- a/doc/manuals/osmogbproxy-usermanual.adoc
+++ b/doc/manuals/osmogbproxy-usermanual.adoc
@@ -23,7 +23,7 @@
 
 include::{srcdir}/chapters/gbproxy-sgsnpool.adoc[]
 
-include::./common/chapters/gb-variants.adoc[]
+include::./common/chapters/gb-ns2.adoc[]
 
 include::./common/chapters/gb.adoc[]