build.zig.zon example broken

[marler8997]

By referencing master the archive will change every time new commits are pushed to master in this repo meaning that if someone added that to their build.zig.zon file the dependency (and their corresponding hash) would become out of sync. I'd recommend the example showing how to pin to a specific sha like this:

.dependencies = .{
    .string = .{
        .url = "https://github.com/JakubSzark/zig-string/archive/PUT_COMMIT_SHA_HERE.tar.gz",
        //the correct hash will be suggested by zig
    }
}