docs: update configService code example

[devniel]

PR Type

Small docs update.

What is the current behavior?

In the Async Options, an example with ConfigModule is being used where configServices uses getString that now should be get.

JwtModule.registerAsync({
  imports: [ConfigModule],
  useFactory: async (configService: ConfigService) => ({
    secret: configService.getString('SECRET'),
  }),
  inject: [ConfigService],
}),