[MCL-2957] Forgotten tweak class in the a1.2.6.json

Mojang forgot the tweak class in the a1.2.6.json

This is the original line in the a1.2.6.json:

"minecraftArguments": "${auth_player_name} ${auth_session} --gameDir ${game_directory} --assetsDir ${game_assets}",

This is wrong.
It should be:

"minecraftArguments": "${auth_player_name} ${auth_session} --gameDir ${game_directory} --assetsDir ${game_assets} --tweakClass net.minecraft.launchwrapper.AlphaVanillaTweaker",

When you join in a multiplayer server, your name is Player524.