Tiny format change of Dockerfile.chiseled-composite (#6083)

Author: simonkurtz-MSFT

samples/aspnetapp/Dockerfile.chiseled-composite

@@ -1,6 +1,6 @@
 # Learn about building .NET container images:
 # https://github.com/dotnet/dotnet-docker/blob/main/samples/README.md
-FROM --platform=$BUILDPLATFORM  mcr.microsoft.com/dotnet/sdk:9.0-noble AS build
+FROM --platform=$BUILDPLATFORM mcr.microsoft.com/dotnet/sdk:9.0-noble AS build
 ARG TARGETARCH
 WORKDIR /source