[python-amaranth-boards-git] fix branch name

Author: yuyichao

archlinuxcn/python-amaranth-boards-git/lilac.yaml

@@ -10,7 +10,7 @@ repo_depends:
 update_on:
   - source: github
     github: amaranth-lang/amaranth-boards
-    branch: master
+    branch: main
   - alias: python
   - source: manual
     manual: 1