[Piglit] [Minor]. Need to update link to readme on piglit web-site

[Den]

Hi guys. There is really trivial thing, but I think, would be great to 
fix it.

https://piglit.freedesktop.org/ on this web-site there is a link 
https://cgit.freedesktop.org/piglit/tree/README, which actually doesn't 
work (there is no file with a name "README".

Expected result: https://cgit.freedesktop.org/piglit/tree/README.md