Fix the documenation for the installation of the plugin

[gt3389b]

Checklist

• [ x] I have read the [contribution guidelines] (https://github.com/Azure/azure-iot-sdk-node/blob/master/.github/CONTRIBUTING.md).
• [ x] I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • [x ] I edited the corresponding document in the devdoc folder and added or modified requirements.
• I submitted this PR against the correct branch:
  • This pull-request contains code (and maybe docs): it is submitted against the develop branch.
  • [ x] This pull-request contains only docs: it is submitted against the master branch.

Reference/Link to the issue solved with this PR (if any)

Description of the problem

Error in documentation

Description of the solution

Fixed documentation

[msftclas]

Hi @gt3389b, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, MSBOT;