Update README.md

fixed "apre-training" to "a pre-training".

README.md

@@ -9,7 +9,7 @@ license: apache-2.0
 
 # Introduction
 
-UL2 is a unified framework for pretraining models that are universally effective across datasets and setups. UL2 uses Mixture-of-Denoisers (MoD), apre-training objective that combines diverse pre-training paradigms together. UL2 introduces a notion of mode switching, wherein downstream fine-tuning is associated with specific pre-training schemes.
+UL2 is a unified framework for pretraining models that are universally effective across datasets and setups. UL2 uses Mixture-of-Denoisers (MoD), a pre-training objective that combines diverse pre-training paradigms together. UL2 introduces a notion of mode switching, wherein downstream fine-tuning is associated with specific pre-training schemes.
 
 ![model image](https://raw.githubusercontent.com/google-research/google-research/master/ul2/figs/ul2.png)