Clarify repository cleanup process

Fix heading - Purging files from repo history doesn't reduce storage.

doc/user/project/repository/reducing_the_repo_size_using_git.md

@@ -23,9 +23,9 @@ Rewriting repository history is a destructive operation. Make sure to back up yo
 you begin. The best way back up a repository is to
 [export the project](../settings/import_export.md#exporting-a-project-and-its-data).
 
-## Purge files from repository history and storage
+## Purge files from repository history
 
-To reduce the size of your repository in GitLab, you must remove references to large files from branches, tags, *and*
+To reduce the size of your repository in GitLab, you must first remove references to large files from branches, tags, *and*
 other internal references (refs) that are automatically created by GitLab. These refs include:
 
 - `refs/merge-requests/*` for merge requests.