Add missing task links (#2449)

lm_eval/tasks/README.md

@@ -73,7 +73,8 @@
 | medqa | Multiple choice question answering based on the United States Medical License Exams. | |
 | [mgsm](mgsm/README.md) | Benchmark of multilingual grade-school math problems. | Spanish, French, German, Russian, Chinese, Japanese, Thai, Swahili, Bengali, Telugu |
 | [minerva_math](minerva_math/README.md) | Mathematics-focused tasks requiring numerical reasoning and problem-solving skills. | English |
-| mmlu | Massive Multitask Language Understanding benchmark for broad domain language evaluation. Several variants are supported. | English |
+| [mmlu](mmlu/README.md) | Massive Multitask Language Understanding benchmark for broad domain language evaluation. Several variants are supported. | English |
+| [mmlu_pro](mmlu_pro/README.md) | A refined set of MMLU, integrating more challenging, reasoning-focused questions and expanding the choice set from four to ten options. | English |
 | [mmlusr](mmlusr/README.md) | Variation of MMLU designed to be more rigorous. | English |
 | model_written_evals | Evaluation tasks auto-generated for evaluating a collection of AI Safety concerns. | |
 | [mutual](mutual/README.md) | A retrieval-based dataset for multi-turn dialogue reasoning. | English |