Fix typo in "build-an-android-border-router.md" am: f286919f80 am: d37846214e

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

Change-Id: Ie84ffdcf3a22218936b89a83861ac15e45da429e
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>