detect/filename: fix buffer description

[regit]

Fix some buffer descriptions

• [x] I have read the contributing guide lines at https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Contributing
• [x] I have signed the Open Information Security Foundation contribution agreement at https://suricata.io/about/contribution-agreement/
• [x] I have updated the user guide (in doc/userguide/) to reflect the changes made (if applicable)

Link to redmine ticket:

Describe changes:

• Fix string in detect filename
• Improve string in file data

[suricata-qa]

Information: QA ran without warnings.

Pipeline 8331

[suricata-qa]

Information: QA ran without warnings.

Pipeline 8331

[victorjulien]

Merged in #8137, thanks!