Update README.md (#3070)

Wrong task name: mmlu_generation doesn't non exist -> mmlu_generative is the correct one

lm_eval/tasks/mmlu/README.md

@@ -36,11 +36,11 @@ Note: The `Flan` variants are derived from [here](https://github.com/jasonwei20/
 
 * `mmlu`: `Original multiple-choice MMLU benchmark`
 * `mmlu_continuation`: `MMLU but with continuation prompts`
-* `mmlu_generation`: `MMLU generation`
+* `mmlu_generative`: `MMLU generation`
 
 MMLU is the original benchmark as implemented by Hendrycks et al. with the choices in context and the answer letters (e.g `A`, `B`, `C`, `D`) in the continuation.
 `mmlu_continuation` is a cloze-style variant without the choices in context and the full answer choice in the continuation.
-`mmlu_generation` is a generation variant, similar to the original but the LLM is asked to generate the correct answer letter.
+`mmlu_generative` is a generation variant, similar to the original but the LLM is asked to generate the correct answer letter.
 
 
 #### Subgroups