[docs] Fix generate-olm-config.js script in dev_environment docs

Summary: filename suffix should've been .js not .json

Test Plan: didn't work before, now it does work

Reviewers: palys-swm, ashoat, benschac, yayabosh

Reviewed By: ashoat, yayabosh

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3621