Use 'origin' as the default remote name (#755)

We use `origin` as the default remote name elsewhere in this doc, so
let's use it here as well?

Author: sharkdp

CONTRIBUTING.md

@@ -138,7 +138,7 @@ Releases can only be performed by Astral team members.
 
 Preparation for the release is automated.
 
-1. Checkout the `main` branch and run `git pull upstream main --recurse-submodules --tags`.
+1. Checkout the `main` branch and run `git pull origin main --recurse-submodules --tags`.
 
 1. Create and checkout a new branch for the release.