Sqlite3 gem not supported on Heroku

When I tried to deploy second part of RoR tutorial to Heroku I get this error:

An error occurred while installing sqlite3 (1.3.5), and Bundler cannot continue.  
Make sure that `gem install sqlite3 -v '1.3.5'` succeeds before bundling.
!  Failed to install gems via Bundler.
! Detected sqlite3 gem which is not supported on Heroku.
!  http://devcenter.heroku.com/articles/how-do-i-use-sqlite3-for-development
!  Heroku push rejected, failed to compile Ruby/rails app
To git@heroku.com:thawing-beyond-7283.git
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'git@heroku.com:thawing-beyond-7283.git'

I searched a little bit about this error and find this stackoverflow post. But when I look more carefully on the Gemfile syntax I found a mistake. I used simply:

But I should have:

As RoR tutorial states.

Piotr Król
Founder and Embedded Systems Consultant at 3mdeb as well as freelance CTO of Vitro Technology and CEO of LPN Plant. Passionate about building firmware that enables advanced hardware features in modern products. Dedicated to customers that treat embedded software security and upgradeability as forethought. Open source firmware evangelist interested in platform security and trusted computing. In favor of fixed price projects with a clear definition of success.