Fix link to Riot Android on mobile guide

[aaronraimist]

Currently links to the old Riot Android im.vector.alpha https://riot.im/app/mobile_guide

[turt2live]

Fuuuuuuuuu--

[ara4n]

i have manually tweaked the live deployment on riot.im/app/mobile_guide for this, and also fixed riot-web/develop in 03b2766

[turt2live]

Have also cherry picked it (ee0b7e9) so it lands in the next release