Fix exploration achievements grammar

[eleriaqueen]

• "The mystery surrounding..." is a correct phrase though not appropriate in this case. But "Treasure surrounding ..." is wrong. I propose the word 'in' instead of "surrounding".
• "Treasure" is uncountable in this case because it's an unknown, both in what it actually is, and the amount of it, so "the treasure" is incorrect because it implies this treasure is well known and very specific.