GitBook: [#13] fixed link

Author: jonasjabari

docs/SUMMARY.md

@@ -2,7 +2,7 @@
 
 * [Welcome](README.md)
 * [Migrating to 3.0](migrate-to-3.0.md)
-* [Back to 2.1 (via core)](https://docs.matestack.io/matestack-ui-bootstrap/v/2.1.0)
+* [Back to 2.1 (via Core)](https://docs.matestack.io/matestack-ui-core/v/2.1.0)
 
 ## Getting started