Update data_source_yaml_to_json.md

[ravi-mrk]

fix for Interpolation

[ashald]

This data source works only starting with Terraform v0.12 and therefore example is HCL2 that is default since that version. Given that it's impossible to use it with earlier versions of Terraform I don't think converting the the example to HCL1 would be of any benefit.

[ravi-mrk]

I was executing these on Terraform v0.11.11 and with these interpolations in place, it was working for me. I just realised after your comments that my terraform version is out of date. Just upgraded to v0.12.1 and this is working with both the versions with interpolation.