Add mark up to code in README, fix custom example

Problem/Motivation

The code examples in the README are not marked up as code, so the formatting breaks down:

https://git.drupalcode.org/project/simplei/-/tree/3.0.x

Also, the custom example ($settings['simple_environment_anonymous'] = "body:after { ...) is missing index, so the current example does not work.

We should make the 3.x branch the default on GitLab, 1.x is a very old version: https://git.drupalcode.org/project/simplei/

Steps to reproduce

1. Visit GitLab and get 1.x version:
   https://git.drupalcode.org/project/simplei/
2. Visit the README for 3.x and see the lines are jumbled up, and that the custom example at the bottom does not work:
   https://git.drupalcode.org/project/simplei/-/tree/3.0.x

Proposed resolution

Remaining tasks

1. Format the code as code.
2. Add index in the example ($settings['simple_environment_anonymous'] = "body:after { ...)
3. Set the default branch to 3.x (currently the old 1.x)

User interface changes

API changes

Data model changes