link to example in docs is invalid

[cmatKhan]

singularity-compose/docs/README.md

Line 30 in e301c95

And a complete example is provided [here](https://github.com/singularityhub/singularity-compose-examples/blob/master/rstudio-simple/singularity-compose.yml).

In the docs, the link to the example is no longer valid

[vsoch]

Here is the updated link - feel free to open a PR to fix! https://github.com/singularityhub/singularity-compose-examples/tree/master/v1.0/rstudio-simple

Note that is also for version 1 of the spec, and we are on version 2. I don't believe that example has been updated.