Fix typo
Improve natural language statements of axioms and definitions.

Justifications for improvements:

A2 - "contains" presupposes a set-theoretic interpretration for properties

A4 - it is intuitively strange to talk about properties "being" (existing), even though technically that's what we do with Henkin semantics. I belive my reformulation is less strange in this respect.

Natural language text is more "natural" when it doesn't contain logical variables and it can also be more easily appreciated by readers who are not formal logicians.