tx config: Update slug for mobile.
tx config: Update slug for mobile.

In Transifex I renamed the resource for the mobile app's strings
from a slug of `messages` to `mobile`, which seems more transparent.
Update this config accordingly.

brainwane/zulip,synicalsyntax/zulip,hackerkid/zulip,kou/zulip,shubhamdhama/zulip,rht/zulip,shubhamdhama/zulip,rishig/zulip,kou/zulip,zulip/zulip,eeshangarg/zulip,tommyip/zulip,dhcrzf/zulip,punchagan/zulip,synicalsyntax/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,showell/zulip,zulip/zulip,andersk/zulip,brainwane/zulip,shubhamdhama/zulip,synicalsyntax/zulip,jackrzhang/zulip,punchagan/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip,dhcrzf/zulip,punchagan/zulip,showell/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,rishig/zulip,zulip/zulip,zulip/zulip,tommyip/zulip,tommyip/zulip,zulip/zulip,synicalsyntax/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,timabbott/zulip,showell/zulip,timabbott/zulip,shubhamdhama/zulip,rht/zulip,timabbott/zulip,jackrzhang/zulip,dhcrzf/zulip,zulip/zulip,shubhamdhama/zulip,brainwane/zulip,eeshangarg/zulip,rishig/zulip,rht/zulip,tommyip/zulip,andersk/zulip,rishig/zulip,tommyip/zulip,shubhamdhama/zulip,rht/zulip,brainwane/zulip,punchagan/zulip,hackerkid/zulip,jackrzhang/zulip,timabbott/zulip,kou/zulip,kou/zulip,jackrzhang/zulip,dhcrzf/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,jackrzhang/zulip,timabbott/zulip,jackrzhang/zulip,showell/zulip,brainwane/zulip,rishig/zulip,hackerkid/zulip,hackerkid/zulip,synicalsyntax/zulip,hackerkid/zulip,jackrzhang/zulip,andersk/zulip,hackerkid/zulip,shubhamdhama/zulip,dhcrzf/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,rishig/zulip,tommyip/zulip,dhcrzf/zulip,dhcrzf/zulip,showell/zulip,synicalsyntax/zulip,timabbott/zulip,brainwane/zulip,rht/zulip,rht/zulip,zulip/zulip,showell/zulip,showell/zulip,eeshangarg/zulip,tommyip/zulip,rishig/zulip