factory_ufs: Align keyword casing

Change the keyword 'as' to upper case.

BUG=b:424375985
TEST=1) ./setup/cros_docker.sh build && ./setup/cros_docker.sh run
     2) docker exec -it ${container_id} bash
     3) (inside docker) /usr/local/factory/bin/factory_ufs -h

Change-Id: Iddefdb9b7879dc1354929ffb56f0b379d462673d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/factory_installer/+/7380077
Tested-by: Po-Heng Chen <pohengchen@google.com>
Reviewed-by: Ruei-fong Lyu <rueifong@google.com>
Commit-Queue: Ruei-fong Lyu <rueifong@google.com>
Auto-Submit: Po-Heng Chen <pohengchen@google.com>

rust/Dockerfile.factory_ufs

diff --git a/rust/Dockerfile.factory_ufs b/rust/Dockerfile.factory_ufs
index f9fc5a2..77897a5 100644
--- a/rust/Dockerfile.factory_ufs
+++ b/rust/Dockerfile.factory_ufs
@@ -2,7 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-FROM rust:alpine as builder
+FROM rust:alpine AS builder
 
 WORKDIR /usr/src/factory_installer
 COPY . .