[docs][server] update dev environment docs to reflect changes to the url json files

Summary: the server now expects two files called squadcal_url.json and commapp_url.json instead of just one app_url.json file. this diff updates the dev environment setup doc to reflect this change.

Test Plan: these are the instructions I followed to build the server locally.

Reviewers: atul, palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, palys-swm, Adrian, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3528