chore: replace xenial with focal in README.md (#885)

Following the steps in the `README.md` to the letter leads to downloading the docker image twice. Once for xenial (which is no longer supported by Netlify builds) and once for focal (in Step 2, once `./test-tools/start-image.sh path/to/site/repo` is executed).

This commit removes mention of the `netlify/build:focal` image to align this with step two.

Author: toniedzwiedz

README.md

@@ -27,7 +27,7 @@ Emulating Netlify's buildbot on your local machine requires the following:
 Open your Docker terminal, and run the following command to pull the default image:
 
 ```
-docker pull netlify/build:xenial
+docker pull netlify/build:focal
 or
 docker pull netlify/build:v3.0.2 # replace the version with a git tag of the specific version you want to test
 ```