Fix typo (#1207)

Fixes annoying warning seen here:
https://github.com/aws-observability/aws-otel-java-instrumentation/pull/1206/files#diff-dd2c0eb6ea5cfc6c4bd4eac30934e2d5746747af48fef6da689e85b752f39557

By submitting this pull request, I confirm that my contribution is made
under the terms of the Apache 2.0 license.

Author: thpierce

Dockerfile

@@ -13,7 +13,7 @@
 # permissions and limitations under the License.
 
 # Stage 1: Build the cp-utility binary
-FROM public.ecr.aws/docker/library/rust:1.89 as builder
+FROM public.ecr.aws/docker/library/rust:1.89 AS builder
 
 WORKDIR /usr/src/cp-utility
 COPY ./tools/cp-utility .