Fix BFG docs on deleting files and directories

What does this MR do?

Fixes BFG documentation, updates bfg command example for removing large files/folders.

Documentation on Using the BFG Repo-Cleaner (source) include a command example that will error out.

$ bfg --delete-files confidential/keys.txt
Error: *** Can only match on filename, NOT path *** - remove '/' path segments
bfg 1.13.0

BFG developer mentions this is expected behavior on StackOverflow.

MR update the documentation to match current command examples as found in the upstream documentation.

bfg --delete-files file.txt and bfg --delete-folders directory/

Related issues

Originally pointed out here: gitlab-foss#19376 (comment 136792547)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the ~Documentation label.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Optional: Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set and that you merge the equivalent EE MR before the CE MR if both exist.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.