[docs] Add step about gem $PATH for dev environment instructions
[docs] Add step about gem $PATH for dev environment instructions

Test Plan: N/A

Reviewers: subnub, palys-swm

Reviewed By: palys-swm

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D640

Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal,Ashoat/squadcal