Fix challenge code for tutorial 3 crashing

Only one of the pipelines were actually adjusted, so let's fix that.

code/beginner/tutorial3-pipeline/src/challenge.rs

@@ -118,12 +118,12 @@ impl State {
                 layout: Some(&render_pipeline_layout),
                 vertex: wgpu::VertexState {
                     module: &shader,
-                    entry_point: "main",
+                    entry_point: "vs_main",
                     buffers: &[],
                 },
                 fragment: Some(wgpu::FragmentState {
                     module: &shader,
-                    entry_point: "main",
+                    entry_point: "fs_main",
                     targets: &[wgpu::ColorTargetState {
                         format: config.format,
                         blend: Some(wgpu::BlendState::REPLACE),