fix run_exe_unit_tests ref

Signed-off-by: Rui Chen <[email protected]>

course/engineering/build-system.md

@@ -403,7 +403,7 @@ pub fn build(b: *std.Build) void {
     const run_exe_unit_tests = b.addRunArtifact(exe_unit_tests); // [!code focus]
 
     // 如果想要跳过外部来自于其他包的单元测试（例如依赖中的包），可以使用 skip_foreign_checks // [!code focus]
-    run_unit_tests.skip_foreign_checks = true; // [!code focus]
+    run_exe_unit_tests.skip_foreign_checks = true; // [!code focus]
 
     // 构建一个 step，用于执行测试 // [!code focus]
     const test_step = b.step("test", "Run unit tests"); // [!code focus]