doc: change suite to trusty in gitian-building.md

doc/gitian-building.md

@@ -320,7 +320,7 @@ Execute the following as user `debian`:
 
 ```bash
 cd gitian-builder
-bin/make-base-vm --lxc --arch amd64 --suite precise
+bin/make-base-vm --lxc --arch amd64 --suite trusty
 ```
 
 There will be a lot of warnings printed during the build of the image. These can be ignored.