[docs] Changing titles for "Running iOS" and "Running Android" to be more specific

Summary: I felt that the previous "Running iOS/Android" titles were not specific enough. Now that there are sections for running the mobile app on iOS and Android physical devices, I think it's a good idea to be more specific here.

Test Plan: N/A

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D2624