Update Sources/SwiftGraph/Graph.swift

Update documentation as suggested.

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

Author: sbeitzel

Sources/SwiftGraph/Graph.swift

@@ -364,7 +364,7 @@ extension Graph {
     /// - parameter fromIndex: the index of the starting vertex
     /// - parameter toIndex: the index of the destination vertex
     /// - parameter visited: a set of vertex indices which will be considered to have been visited already
-    /// - parameter reachability: a dictionary where the key is a vertex index and the value is whether or not a path exists from that vertex to the destination
+    /// - parameter reachability: a dictionary mapping vertex indices to a Boolean indicating whether a path exists (`true`) or does not exist (`false`) from that vertex to the destination
     /// - returns: the number of paths that exist going from the start to the destinatin
     func countPaths(fromIndex startIndex: Int,
                     toIndex endIndex: Int,