Fix CockroachDB tutorial's link being relative to 'GETTING_STARTED.md' (

golang-migrate#819)

* Fix link root

Fix the link root so that it doesn't point relatively.

* Simplify to just the / root

database/cockroachdb/TUTORIAL.md

@@ -50,7 +50,7 @@ And in the `.down.sql` let's delete it:
 ```
 DROP TABLE IF EXISTS example.users;
 ```
-By adding `IF EXISTS/IF NOT EXISTS` we are making migrations idempotent - you can read more about idempotency in [getting started](GETTING_STARTED.md#create-migrations)
+By adding `IF EXISTS/IF NOT EXISTS` we are making migrations idempotent - you can read more about idempotency in [getting started](/GETTING_STARTED.md#create-migrations)
 
 ## Run migrations
 ```
@@ -139,4 +139,4 @@ func main() {
 	}
 }
 ```
-You can find details [here](README.md#use-in-your-go-project)
+You can find details [here](README.md#use-in-your-go-project)