Fix TypeScript error in SQL docs for UsersService

The function type return `findOne` of UsersService seems incorrect.

The line `this.usersRepository.findOneBy({ id })` returns a `User` or `null` if no user with the matching id is found. 
However, the function return is `Promise<User>`.

Also, the parameter `id` of the `remove` function for UsersService is a `string` which is inconsistent with all the other functions of UsersService where the parameter `id` is a `number`

Author: HugoMichard

content/techniques/sql.md

@@ -180,11 +180,11 @@ export class UsersService {
     return this.usersRepository.find();
   }
 
-  findOne(id: number): Promise<User> {
+  findOne(id: number): Promise<User | null> {
     return this.usersRepository.findOneBy({ id });
   }
 
-  async remove(id: string): Promise<void> {
+  async remove(id: number): Promise<void> {
     await this.usersRepository.delete(id);
   }
 }