docs(README): format code blocks for SSH key generation instructions

README.md

@@ -8,7 +8,7 @@
 
 参见 [flux/README.md](flux/README.md)
 
-`
+```shell
 ssh-keygen -t ed25519 -C "flux" -f ./flux-git-auth -N ""
 
 ssh-keyscan github.com > ./known_hosts
@@ -17,4 +17,4 @@ kubectl -n infra-gitops create secret generic flux-git-auth \
  --from-file=identity=./flux-git-auth \
  --from-file=identity.pub=./flux-git-auth.pub \
  --from-file=known_hosts=./known_hosts
-`
+```