As pointed out by Pavel Šimerda, you can simply do
git push
I've added the specification for a 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 [ […]]
...
The format of a parameter is…the source ref , followed by
a colon :, followed by the destination ref …
The tells which ref on the remote side is updated with this push…If
: is omitted, the same ref as will be updated…
tag means the same as refs/tags/:refs/tags/.