Manga District: fix thumbnail

[vetleledaal]

Closes #3126

Checklist:

• [x] Updated extVersionCode value in build.gradle for individual extensions
• [x] Updated overrideVersionCode or baseVersionCode as needed for all multisrc extensions
• [x] Referenced all related issues in the PR body (e.g. "Closes #xyz")
• [x] Added the isNsfw = true flag in build.gradle when appropriate
• [x] Have not changed source names
• [x] Have explicitly kept the id if a source's name or language were changed
• [x] Have tested the modifications by compiling and running the extension through Android Studio
• [x] Have removed web_hi_res_512.png when adding a new extension