Fix Coordinate::NEIGHBOURS constant

It currently didn't actually remove [0, 0]. That was
a mistake but it was self corrected by other checks
later which is why it still works correctly.

Fixing this will make neighbours iteration a bit faster.

lib/minesweeper/coordinate.rb

@@ -1,6 +1,6 @@
 module Minesweeper
   Coordinate = Data.define(:x, :y) do
-    NEIGHBOURS = (Enumerator.product([-1, 0, 1], [-1, 0, 1]).to_a - [0, 0]).map { |x, y| self.new(x, y) }
+    NEIGHBOURS = (Enumerator.product([-1, 0, 1], [-1, 0, 1]).to_a - [[0, 0]]).map { |x, y| self.new(x, y) }
 
     def neighbour?(other)
       [(self.x - other.x).abs, (self.y - other.y).abs].max <= 1