Update SVN import link
What does this MR do and why?
When using Repository URL as the import source for a project, a help text directs the user to refer to the documentation if using SVN:
The current link directs to a non-existent docs page, and no redirect applies. This can result in a 404
response on self-managed instances. This merge request fixes this behaviour by adding a direct anchor link to the new location of SVN information at: https://docs.gitlab.com/ee/user/project/import/index.html#import-repositories-from-subversion
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