Clarify `associative?` vector function definition

[k4y3ff]

In data_structures.md, the definition of the associative? function for vectors states,

It lets us know whether we can look up values in this collection using keys. We can with a vector; each item in the vector has a key starting with zero and going up that points to it. In our vector [:a :b :c], the key 0 points to :a, the key 1 points to :b, and the key 2 points to :c.

However, at this point in the curriculum, we have not yet described what a map is, nor have we defined what a key is, so this definition is problematic.

[cndreisbach]

I've removed all references to associative? as they aren't necessary for the curriculum.