docs: fix invalid docker-buildx.sh usage example in README and README_ZH (#7570)

docs: fix invalid docker-buildx.sh usage in README and README_ZH

Replace the docker-buildx.sh --build-arg RELEASE=latest example, which
the script never accepted as a CLI flag (--build-arg is only used
internally for docker buildx build), with the supported invocations:

- bare ./docker-buildx.sh for the default local build
- ./docker-buildx.sh -p linux/amd64 for a single-platform local build

Also update the surrounding comments to reflect single-platform local
builds and add the multi-arch example comment accordingly.

README.md

@@ -211,7 +211,10 @@ For developers who want to build RustFS Docker images from source with multi-arc
 
 ```bash
 # Build multi-architecture images locally
-./docker-buildx.sh --build-arg RELEASE=latest
+./docker-buildx.sh
+
+# Build a single-platform image locally
+./docker-buildx.sh -p linux/amd64
 
 # Build and push to registry
 ./docker-buildx.sh --push

README_ZH.md

@@ -150,7 +150,10 @@ docker compose -f docker-compose-simple.yml up -d
 
 ```bash
 # 在本地构建多架构镜像
-./docker-buildx.sh --build-arg RELEASE=latest
+./docker-buildx.sh
+
+# 在本地构建单平台镜像
+./docker-buildx.sh -p linux/amd64
 
 # 构建并推送到仓库
 ./docker-buildx.sh --push