Fix kernel branch name in README

Test: follow the instruction
Change-Id: If39f15a93961128e5bffcfee26394e09f4435ef2
diff --git a/microdroid/kernel/README.md b/microdroid/kernel/README.md
index 44f828c..12d8770 100644
--- a/microdroid/kernel/README.md
+++ b/microdroid/kernel/README.md
@@ -10,7 +10,7 @@
 ### Checkout the GKI source code.
 
 ```bash
-repo init -u https://android.googlesource.com/kernel/manifest -b android14-5.15
+repo init -u https://android.googlesource.com/kernel/manifest -b common-android14-5.15
 repo sync
 ```