add mi439 twrp name

* capitalization matters
* changes untested
* partially fixes #352

Author: SirRGB

openandroidinstaller/assets/configs/Mi439.yaml

@@ -15,6 +15,7 @@ metadata:
     - olive
     - olivelite
     - olivewood
+  twrp-link: mi439
   notes:
     - Be careful when choosing OrangeFox version, Android 12 & 13 ROM needs OrangeFox version code with `A12`, for example `R11.1_5_A12`. Android 10 & 11 ROM needs OrangeFox version code without `A12` (bellow on the page)
 requirements: