Change link to use valid Concourse documentation
It is currently pointing to which is not valid. So changed it to use /index.html
@@ -4,7 +4,7 @@ A command line interface that runs a build in a container with [ATC](https://git
[The documentation](https://concourse-ci.org/fly.html) is hosted together with Concourse's docs.
-A good place to start learning about Concourse is [its documentation](https://concourse-ci.org/docs.html)
+A good place to start learning about Concourse is [its documentation](https://concourse-ci.org/index.html)
or its [BOSH release](https://github.com/concourse/concourse).
## Reporting Issues and Requesting Features
