Update db migration

docker/backend/db/migrations/20220205143534_init.sql

@@ -17,12 +17,12 @@
  */
 -- +micrate Up
 -- SQL in section ' Up ' is executed when this migration is applied
-CREATE TYPE user_roles AS ENUM ('Teacher', 'Student');
+CREATE TYPE user_role AS ENUM ('Teacher', 'Student');
 
 CREATE TABLE users(
   id BIGSERIAL PRIMARY KEY,
   username TEXT UNIQUE NOT NULL,
-  role user_roles NOT NULL,
+  role user_role NOT NULL,
   admin BOOLEAN NOT NULL
 );