Update README.md (#6)

Summary:
a an -> an

Pull Request resolved: #6

Reviewed By: sashafrolov

Differential Revision: D52010791

fbshipit-source-id: be077de922ac7247a3c79ad042db19e4032eff5b

README.md

@@ -10,7 +10,7 @@
 
 # Purple Llama
 
-Purple Llama is a an umbrella project that over time will bring together tools
+Purple Llama is an umbrella project that over time will bring together tools
 and evals to help the community build responsibly with open generative AI
 models. The initial release will include tools and evals for Cyber Security and
 Input/Output safeguards but we plan to contribute more in the near future.