Fix Vale rule links in styleguide

What does this MR do?

This fixes broken Vale rule links in the documentation guideline.

❯ rg "blob/master/doc/\.vale/gitlab/" doc --max-columns 10000
doc/development/documentation/styleguide/index.md
145:- Write in US English with US grammar. (Tested in [`British.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/.vale/gitlab/British.yml).)
378:- Use serial (Oxford) commas before the final **and** or **or** in a list of three or more items. (Tested in [`OxfordComma.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/.vale/gitlab/OxfordComma.yml).)
384:- Use one space between sentences. (Use of more than one space is tested in [`SentenceSpacing.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/.vale/gitlab/SentenceSpacing.yml).)
392:- `“` `”` `‘` `’`: Double or single typographer's ("curly") quotation marks. Use straight quotes instead. (Tested in [`NonStandardQuotes.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/.vale/gitlab/NonStandardQuotes.yml).)
830:- Put the entire link on a single line, even if the link is very long. ([Vale](../testing/vale.md) rule: [`SubstitutionWarning.yml`](https://gitlab.com/gitlab-org/gitlab/-/blob/master/doc/.vale/gitlab/MultiLineLinks.yml)).

Related issues

Author's checklist

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Reviewer's checklist

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