known-issues: update fastgit.org URL

Signed-off-by: Stephen Gutekanst <stephen@hexops.com>

content/about/known-issues.md

@@ -75,7 +75,7 @@ LD_PRELOAD=/run/current-system/profile/lib/libvulkan.so zig-out/bin/gpu-hello-tr
 
 **Background**: `zig build` on the first time around will download a 20-30MB file of Dawn (Google's WebGPU implementation) from https://github.com/hexops/mach-gpu-dawn/releases - the build system uses `curl` to do this automatically. You can of course build Dawn from source using the `-Ddawn-from-source=true` flag, but this will require a clone of the Dawn sources which are a larger download and takes a few minutes to build as it is a large C++ codebase.
 
-Users in Chinese mainland may find that download speeds to github.com are too slow (hours to download a 30 MB file), and apparently it is common to use GitHub mirror sites like https://fastgit.org to download files from GitHub.
+Users in Chinese mainland may find that download speeds to github.com are too slow (hours to download a 30 MB file), and apparently it is common to use GitHub mirror sites like https://doc.fastgit.org to download files from GitHub.
 
 You can have Mach use such websites by setting an environment variable e.g.: