Update install.md to add correct board type

[iayanpahwa]

Change board from nrf52840dk to nrf52840dk_opensk

Fixes #650

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[ia0]

Thanks for the PR but the stable branch does not have an nrf52840dk_opensk board. This is specific to the develop branch. And the documentation there is up-to-date. By default on Github shows you the stable branch which may be confusing.