[docs] Fix cp command in dev_environment instructions
Summary: Noticed this when adjusting my environment, straightforward change.
Test Plan:
- Ran the existing command, it failed. (Since there's no longer a server directory)
- Ran the new command, it worked.
Reviewers: benschac, ashoat, palys-swm
Reviewed By: benschac, ashoat
Subscribers: Adrian, karol-bisztyga, yayabosh
Differential Revision: https://phabricator.ashoat.com/D3619