unify readme code blocks

README.md

@@ -6,7 +6,7 @@ an automatically schema-versioned content management system
 
 prepare test data:
 
-```
+```shell
 cp -r samples/blog .
 cd blog
 git init
@@ -16,14 +16,14 @@ git commit -m 'first commit'
 
 run frontend:
 
-```console
+```shell
 cd frontend
 reload
 ```
 
 run backend:
 
-```console
+```shell
 cd backend
 cabal run backend -- serve
 ```