[PYSPARK] Fix a typo in "fold" function in rdd.py

This will make the “fold” function consistent with the "fold" in
rdd.scala and other "aggregate" functions where “acc” goes first.
Otherwise, users have to write a lambda function like “lambda x, y:
op(y, x)” if they want to use “zeroValue” to the result type.

Author: Alain

python/pyspark/rdd.py

@@ -827,7 +827,7 @@ def fold(self, zeroValue, op):
         def func(iterator):
             acc = zeroValue
             for obj in iterator:
-                acc = op(obj, acc)
+                acc = op(acc, obj)
             yield acc
         vals = self.mapPartitions(func).collect()
         return reduce(op, vals, zeroValue)