Link on readme is to Spring21 instead of latest

[fayolle]

The link at the end of the readme file
https://github.com/mitmath/computational-thinking/blob/578736f39c6eeb388ff5c51f9f28f0b8d8f29670/README.md
points to the Spring21 version instead of the latest version.

# Go to [computationalthinking.mit.edu](https://computationalthinking.mit.edu/Spring21/) to learn more!

should be

# Go to [computationalthinking.mit.edu](https://computationalthinking.mit.edu/) to learn more!

[fonsp]

Thanks!