Installing Ruby: Update section of the instructions with rbenv init

[beneficial-radish]

Checks

• This is not a duplicate of an existing issue (please have a look through our open issues list to make sure)
• I have thoroughly read and understand The Odin Project Contributing Guide
• Would you like to work on this issue?

Describe your suggestion

This is my first time raising an issue or contributing, and I unfortunately went a bit out of order. However, this issue was raised previously and worked on, and it seems that they accidentally got edited back out or didn't merge properly or something.

Path

Ruby / Rails

Lesson Url

https://www.theodinproject.com/lessons/ruby-installing-ruby

(Optional) Discord Name

beneficial.radish

(Optional) Additional Comments

This is my first pull request/contribution to an Open Source project ever! I'm honored that it was with a project from which I've benefited so much.

[beneficial-radish]

Linking the closed issue here as the thread is still active and providing relevant insights.
#28436