Fix set minus in docs (#157)

* Fix Unicode in set_operations docs

* Fix another setminus in docs

Author: dpsanders

src/intervals/set_operations.jl

@@ -87,7 +87,7 @@ union(a::Interval, b::Interval) = hull(a, b)
 """
     setdiff(x::Interval, y::Interval)
 
-Calculate the set difference `x \ y`, i.e. the set of values
+Calculate the set difference `x ∖ y`, i.e. the set of values
 that are inside the interval `x` but not inside `y`.
 
 Returns an array of intervals.

src/multidim/setdiff.jl

@@ -36,7 +36,7 @@ end
 """
     setdiff(A::IntervalBox{N,T}, B::IntervalBox{N,T})
 
-Returns a vector of `IntervalBox`es that are in the set difference `A \ B`,
+Returns a vector of `IntervalBox`es that are in the set difference `A ∖ B`,
 i.e. the set of `x` that are in `A` but not in `B`.
 
 Algorithm: Start from the total overlap (in all directions);