Fix AppData errors in 5.0 branch

The MR fixes errors that came up when running the appdata validator and during the flathub build process.

• Changed image url from brusheditor.png to brush_editor.png as the former didn't exist
• Moved tag to near the end on two lines for captions as they were resulting in an error
• Add missing slash in release line

Test Plan

Using appstream-util validate-relax org.kde.krita.appdata.xml.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.