Replace relative url with full path, remove old reference for indexes

Closes #2176 (gitlab.com)

doc/update/mysql_to_postgresql.md

 # Migrating GitLab from MySQL to Postgres
-*Make sure you view this [guide from the `master` branch](../../../master/doc/update/mysql_to_postgresql.md) for the most up to date instructions.*
+*Make sure you view this [guide from the `master` branch](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/update/mysql_to_postgresql.md#migrating-gitlab-from-mysql-to-postgres) for the most up to date instructions.*
 If you are replacing MySQL with Postgres while keeping GitLab on the same server all you need to do is to export from MySQL, convert the resulting SQL file, and import it into Postgres. If you are also moving GitLab to another server, or if you are switching to omnibus-gitlab, you may want to use a GitLab backup file. The second part of this documents explains the procedure to do this.
@@ -70,5 +70,5 @@ sudo -u git -H gzip db/database.sql
 sudo -u git -H tar rf TIMESTAMP_gitlab_backup.tar db/database.sql.gz
 # Done! TIMESTAMP_gitlab_backup.tar can now be restored into a Postgres GitLab
-# installation. Remember to recreate the indexes after the import.
+# installation.
 ```