docs: fix gitlab pages deploy syntax

[jonthesquirrel]

Description

The GitLab Pages example causes GitLab CI to fail due to invalid syntax:

Here is the documentation about the syntax: https://docs.gitlab.com/ee/ci/yaml/index.html#rules

This PR fixes the syntax.

Additional context

This change has been tested to work on GItLab CI.

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

Thanks @jonthesquirrel, you may also want to check if the instructions for https://vitepress.vuejs.org/guide/deploy.html#gitlab-pages-and-gitlab-ci are correct in case you would like to also improve the VitePress docs. I see there isn't a conditional there, but maybe there should be.