Update Multiplayer Server Connection Wording

[krkode]

Proposed changes for playing/multiplayer/index.rst

Currently documentation references previous button text "New Server".

As at OpenRCT2 v0.4.5 the button is labelled "Add Server".

Additionally to join a server the pop up menu option is labelled "Join Game", while current documentation states it is "Join Server".

[krkode]

if approved id be totally down to take up this issue got the pr ready to submit :D

[tupaschoal]

I'm confused about this, didn't find where needs changing