Fix typo error in README
# `imgui-gfx-examples`
-- Run with OpenGL backend: `cargo run --example hello_gfx`
-- Run with DirectX backend: `cargo run --example hello_gfx --features directx --no-default-features`
+- Run with OpenGL backend: `cargo run --example gfx_hello_world`
+- Run with DirectX backend: `cargo run --example gfx_hello_world --features directx --no-default-features`
