Runner should match file name case-sensitively while searching for readme.md

- There are some services under specification folder adding `README.md` to describe its service. This would be mistakenly considered as 'readme.md' to be used for SDK generation. It needs to change the file comparison code to use the original file name when searching files.
Here is an example README.md:
https://github.com/Azure/azure-rest-api-specs/blob/main/specification/communication/Communication.JobRouter/README.md