[docs] Android emulator -> Android Emulator

Summary: Fixing the capitalization in this section to reflect the change made in d2706.

Test Plan: n/a

Reviewers: atul, ashoat

Reviewed By: atul, ashoat

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

Differential Revision: https://phabricator.ashoat.com/D2711