Docs: Add spaces before code snippets in vuex.md

What does this MR do?

I realised while reading the html version of this doc on https://docs.gitlab.com/ee/development/fe_guide/vuex.html that some of the javascript code snippet examples were not rendered correctly.

So I added a space between the text and code snippet so that it will be rendered correctly in the HTML version.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer