Fix typo in "build-an-android-border-router.md" am: f286919f80

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Connectivity/+/3272199

Change-Id: I22079b9469846517c68b8a6e5808475d7a443722
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>