[UPDATE] mathematical notations

Hi,

On an image caption in https://huggingface.co/learn/deep-rl-course/en/unit1/rl-framework, I found the expression

> 'The cumulative reward = rt+1 (rt+k+1 = rt+0+1 = rt+1)+ rt+2 (rt+k+1 = rt+1+1 = rt+2) + ...'.

However, formal mathematical writing would use r_{t+1} for better readability unless LaTeX rendering is supported.