branch off of main in step 2 (#38)

* branch off of main in step 2

be more explicit that you should branch off of `main` in step 2, otherwise we won't update STEP correctly

* stray character 🪓

README.md

@@ -111,7 +111,7 @@ Releases are usually made of many smaller changes. Let's pretend we don't know a
 
 ### :keyboard: Activity: Update `base.css`
 
-1. Create a new branch and change the `body` CSS declaration in `base.css` to match what is below. This will set the page background to black
+1. Create a new branch off of the `main` branch and change the `body` CSS declaration in `base.css` to match what is below. This will set the page background to black
 ```
 body {
     background-color: black;