Correct comment

kdtree/kdtree.pyx

@@ -168,7 +168,7 @@ cdef cypclass KDTree:
 
         # When object are activated (set as Actors), methods
         # are reified. When using those reified methods
-        # a new argument is prepredend for the Promise,
+        # a new argument is prepredend for a predicate,
         # which we aren't using using here, hence the extra NULL.
         self.root.build_node(NULL,
                              self.data_ptr,