As pointed out by Pavel Šimerda, you can simply do
git push <remote> <tag>
I've added the specification for a remote <remote> so that the command doesn't depend on a user's push.default configuration.
Here is a summary of the relevant documentation that explains how to push a specific tag:
git push [<repository> [<refspec>…]]
<refspec>...
The format of a <refspec> parameter is…the source ref <src>, followed by
a colon :, followed by the destination ref <dst>…
The <dst> tells which ref on the remote side is updated with this push…If
:<dst> is omitted, the same ref as <src> will be updated…
tag <tag> means the same as refs/tags/<tag>:refs/tags/<tag>.