install older ruby code example

Example: how to use multiple ruby version in mac as per project

# Install ruby 2.7
rbenv install 2.7.1

# Make ruby 2.7 the default version
$ rbenv global 2.7.1

# Or make 2.7 the default versión only on a specific project
$ cd myproject
$ rbenv local 2.7.1
# this create a ".ruby-version" file