Fix minor typo in docs

doc/update/README.md

-+ [The indivual upgrade guides](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/update)
-+ [Uprader](upgrader.md)
++ [The individual upgrade guides](https://gitlab.com/gitlab-org/gitlab-ce/tree/master/doc/update)
++ [Upgrader](upgrader.md)
 + [Ruby](ruby.md)
 + [Patch versions](patch_versions.md)
 + [MySQL to PostgreSQL](mysql_to_postgresql.md)