Fix config init for branding.disableUsedPercentage

[putty182]

Description
Since #2136, running filebrowser config init shows an error due to case-sensitive typo in config_init.go.

This PR fixes #2576.

🚨 Before submitting your PR, please read community, and indicate which issues (in any of the repos) are either fixed or closed by this PR. See GitHub Help: Closing issues using keywords.

• DO make sure you are requesting to pull a topic/feature/bugfix branch (right side). Don't request your master!
• DO make sure you are making a pull request against the master branch (left side). Also you should start your branch off our master.
• DO make sure that File Browser can be successfully built. See builds and development.
• DO make sure that related issues are opened in other repositories. I.e., the frontend, caddy plugins or the web page need to be updated accordingly.
• AVOID breaking the continuous integration build.

[illnesse]

Has there been any progress on this? It currently breaks all new installs for our cloud customers >.>

ETA: that was quick, thanks <3