There's a convenient little utility called [`symlinks`](https://github.com/brandt/symlinks) (originally by Mark Lords, now maintained by J. Brandt Buckley), present in many Linux distributions.

To convert all the absolute symbolic links in a directory to be relative:

    symlinks -c /path/to/directory

Unfortunately there's no option to convert a single link, you'd have to move it to its own directory first.

    mkdir ../tmp
    mv link-to-relativize ../tmp
    symlinks ../tmp
    mv ../tmp/* .
    rmdir ../tmp