Fix docs with underscore postfix for hyperlink?

[jeroenhe]

Hopefully fixes #1978

I'm sorry this seems to be ending up in a "trial-and-error". I just don't know how to quickly verify changes to .rst files locally.

[cyberw]

Have a look at http://docs.locust.io/en/stable/developing-locust.html#build-documentation

[jeroenhe]

Have a look at http://docs.locust.io/en/stable/developing-locust.html#build-documentation

Thank you. It was actually fairly easy to build the documentation locally. The previous links with :ref: did cause these warnings in the documentation build log output:

Users/jeroen/Src/locust/docs/running-cloud-integration.rst:25: WARNING: undefined label: https://github.com/locustio/locust/blob/master/examples/terraform/aws/readme.md
/Users/jeroen/Src/locust/docs/running-cloud-integration.rst:27: WARNING: undefined label: https://learn.hashicorp.com/tutorials/terraform/install-cli?in=terraform/aws-get-started#install-terraform-on-linux
/Users/jeroen/Src/locust/docs/running-cloud-integration.rst:29: WARNING: undefined label: https://registry.terraform.io/modules/marcosborges/loadtest-distribuited/aws/latest

I can confirm that this PR fixes the links. These warnings are now gone and the links actually works:

[cyberw]

Thx