Disambiguate 急切 (eager) vs 即时 (JIT) execution

Per Copilot translation review: 即时 is used elsewhere in this file
for JIT compilation, so using 即时 again for 'eager execution' was
ambiguous. Use 急切 (Eager) with English parenthetical for clarity.

Author: mmcky

lectures/jax_intro.md

@@ -604,7 +604,7 @@ with qe.Timer():
     y = f(x)
 ```
 
-**即时**执行模型
+**急切**（Eager）执行模型
 
 * 每个操作在遇到时立即执行，在下一个操作开始之前将其结果实体化。