changing game_exists variable name to make the logic more clear

New Features

• What feature(s) did you add?

I fixed a confusingly named variable in the EOSIO example code for tic_tac_toe. tic_tac_toe is used to teach new developers how to use the EOSIO platform, and having bad documentation raises the learning curve.

• How did you implement it/them?

See the diff of the files here:
https://github.com/EOSIO/eos/pull/1419/files