Fix download ESPIn repository example

[Mark Piper]

In the "Download the ESPIn repository" section of the last shell lesson, we ask participants to download the ESPIn repository as a zip archive with the wget command. The problem is that while wget is typically installed on Linux, it's not on macOS or Windows. This example should be fixed to work on all platforms.

Expected behavior: Download repo as zip archive
Actual behavior: Error message is thrown because wget isn't available

[Mark Piper]

The most likely solution is to use curl instead of wget. Note, however, that the URL used in the example performs a redirect that wget handles but curl doesn't. Here's the correct way to do this with curl:

$ curl https://codeload.github.com/csdms/espin/zip/main -o main.zip

[Mark Piper]

Fixed in #32.