docs: check off environment variables task in README

README.md

@@ -55,7 +55,7 @@ Once a task is completed, it should be checked off.
 - [x] Write a VFS driver that provides the contents of the CPIO initial ramdisk to the VFS layer.
 - [x] Create a `hello-world` app. It should print `Hello, World` to its own stdout. The kernel should route this to Qemu and to the VGA dispaly. Ensure this work.
 - [x] Implement the fork system call.
-- [ ] Implement environment variables. Apps should be able to modify this, and it should be copied to new forks of an app.
+- [x] Implement environment variables. Apps should be able to modify this, and it should be copied to new forks of an app.
 - [ ] Create a basic shell program `sh`. This shell must be able to start the hello-world app. It must include `cd` as a built-in to change the current working directory.
 - [ ] Create an `ls` app. It should list the contents of the current working directory, via the environment variable.
 - [ ] Create a devicefs vfs driver. The devicefs subsystem should allow drivers to create new block devices devices.