[docs] replace "simulator" with "virtual device" to be consistent between Android and iOS instruction sets

Summary: After discussion in D2612, came to conclusion that this change must be made as well. Three other sections refer to this explanation, so this change encompasses these cases (simulator, emulator, physical devices).

Test Plan: n/a

Reviewers: atul, ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D2688