Fixed the broken link in model_card.md

[PranithChowdary]

Previous link to the paper (https://d4mucfpksywv.cloudfront.net/better-language-models/language_models_are_unsupervised_multitask_learners.pdf) is broken,

so replacing with new updated link (https://cdn.openai.com/better-language-models/language_models_are_unsupervised_multitask_learners.pdf)