README.md: Update rspec installation instructions

Because

Some learners have experienced issues with rspec failing to run

Adds an instruction to have the user run $ rbenv rehash in the event their rspec gem fails to run

[x] I have thoroughly read and understand The Odin Project Contributing Guide
[x] The title of this PR follows the location of change: brief description of change format, e.g. Data types exercise: Update spec files
[x] The Because section summarizes the reason for this PR
[x] The This PR section has a bullet point list describing the changes in this PR
[ ] If this PR addresses an open issue, it is linked in the Issue section
[ ] If this PR includes changes that needs to be updated on the solutions branch, I have created another PR (and linked it to this PR).