Fix executable name

c/sdl/build.zig

pub fn build(b: *Builder) void {
    const mode = b.standardReleaseOptions();
    const exe = b.addExecutable("sdl-zig-demo", "./hello_sdl.zig");
    const exe = b.addExecutable("hello_sdl", "./hello_sdl.zig");
    exe.setBuildMode(mode);
    exe.linkSystemLibrary("SDL2");
    exe.linkSystemLibrary("SDL2_ttf");