Fix broken tags link

Author: anacrolix

cmd/migrate/README.md

@@ -63,7 +63,7 @@ $ go install -tags 'postgres' github.com/golang-migrate/migrate/v4/cmd/migrate@l
 to build the cli for use with other databases, replace the `postgres` build tag
 with the appropriate database tag(s) for the databases desired.  The tags
 correspond to the names of the sub-packages underneath the
-[`database`](../database) package.
+[`database`](../../database) package.
 1. Similarly to the database build tags, if you need to support other sources, use the appropriate build tag(s).
 1. Support for build constraints will be removed in the future: https://github.com/golang-migrate/migrate/issues/60
 1. For versions of Go 1.15 and lower, [make sure](https://github.com/golang-migrate/migrate/pull/257#issuecomment-705249902) you're not installing the `migrate` CLI from a module. e.g. there should not be any `go.mod` files in your current directory or any directory from your current directory to the root