[Mobile] The initial state of sidebar hamburger is invalid

[nscyclone]

On devices with screen width lower than 768px the sidebar hamburger doesn't respond to the first click.
Steps to reproduce:

• Make your browser window narrower than 768px;
• Visit the index page of the latest stable Elixir docs;
• The hamburger color is initially gray. After the first click it will become dark and only after the second click the sidebar will be toggled.

Example: