Docs feedback: Reduce repo size error

Link the doc and describe what is wrong with it.

For https://docs.gitlab.com/ee/user/project/repository/reducing_the_repo_size_using_git.html it is indicated "Install either git filter-repo or git-sizer using a supported package manager or from source." But in fact

1. this should indicate git-filter-repo rather than git filter-repo
2. git-sizer alone cannot do the job, as the describe procedure below the sentence states, reduce commands are done using git-filter-repo so the sentence should say "Install git-filter-repo and optionally git-sizer using a supported package manager or from source."