Update the name of the sample storage plugin to sample-storage

[Iceber]

clusterpedia-io/clusterpedia#443 (comment)

[Iceber]

$ grep sample-storage-layer . -r
./go.mod:module github.com/clusterpedia-io/sample-storage-layer
./clusterpedia/hack/builder.sh:            eg. CLUSTERPEDIA_REPO=/plugins/sample-storage-layer
./Makefile:STORAGE_PLUGIN ?= sample-storage-layer
./storage/register.go:  StorageName = "sample-storage-layer"
./storage/register.go:  defaultLogFileName = "/var/log/clusterpedia/sample-storage-layer.log"
./README.md:$ STORAGE_PLUGINS=./plugins ./bin/apiserver --storage-name=sample-storage-layer --storage-config=./config.yaml <... other flags>
./README.md:$ git clone --recursive https://github.com/clusterpedia-io/sample-storage-layer.git
./README.md:$ cd sample-storage-layer
./README.md:$ file ./plugins/sample-storage-layer.so
./README.md:./plugins/sample-storage-layer.so: Mach-O 64-bit dynamically linked shared library x86_64
./README.md:$ STORAGE_PLUGINS=./plugins ./bin/apiserver --storage-name=sample-storage-layer --storage-config=./config.yaml <... other flags>
./README.md:$ STORAGE_PLUGINS=./plugins ./bin/clustersynchro-manager --storage-name=sample-storage-layer --storage-config=./config.yaml <... other flags>
./main.go:      plugin "github.com/clusterpedia-io/sample-storage-layer/storage"