fixed links

doc/workflow/importing/README.md

 # Migrating projects to a GitLab instance
 
-1. [Bitbucket](doc/workflow/import_projects_from_bitbucket.md)
-2. [GitHub](doc/workflow/import_projects_from_github.md)
-3. [GitLab.com](doc/workflow/import_projects_from_gitlab_com.md)
-4. [SVN](doc/workflow/migrating_from_svn.md)
+1. [Bitbucket](workflow/import_projects_from_bitbucket.md)
+2. [GitHub](workflow/import_projects_from_github.md)
+3. [GitLab.com](workflow/import_projects_from_gitlab_com.md)
+4. [SVN](workflow/migrating_from_svn.md)
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