Update reference links

Signed-off-by: Burak Sekili <[email protected]>

Author: buraksekili

docs/book/src/cronjob-tutorial/cronjob-tutorial.md

@@ -69,3 +69,6 @@ Otherwise `--repo=<module path>` must be set.
 Read the [Go modules blogpost][go-modules-blogpost] if unfamiliar with the module system.
 
 </aside>
+
+[GOPATH-golang-docs]: https://golang.org/doc/code.html#GOPATH
+[go-modules-blogpost]: https://blog.golang.org/using-go-modules