[hotfix] [docs] Fix ProcessWindowFunction code snippets.

This closes apache#6527.

docs/dev/stream/operators/windows.md

@@ -724,17 +724,19 @@ A `ProcessWindowFunction` can be defined and used like this:
 DataStream<Tuple2<String, Long>> input = ...;
 
 input
-  .keyBy(<key selector>)
-  .window(<window assigner>)
-  .process(new MyProcessWindowFunction());
+ .keyBy(t -> t.f0)
+ .timeWindow(Time.minutes(5))
+ .process(new MyProcessWindowFunction());
 
 /* ... */
 
-public class MyProcessWindowFunction extends ProcessWindowFunction<Tuple<String, Long>, String, String, TimeWindow> {
+public class MyProcessWindowFunction 
+ extends ProcessWindowFunction<Tuple2<String, Long>, String, String, TimeWindow> {
 
- void process(String key, Context context, Iterable<Tuple<String, Long>> input, Collector<String> out) {
+ @Override
+ public void process(String key, Context context, Iterable<Tuple2<String, Long>> input, Collector<String> out) {
  long count = 0;
- for (Tuple<String, Long> in: input) {
+ for (Tuple2<String, Long> in: input) {
  count++;
  }
  out.collect("Window: " + context.window() + "count: " + count);
@@ -749,9 +751,9 @@ public class MyProcessWindowFunction extends ProcessWindowFunction<Tuple<String,
 val input: DataStream[(String, Long)] = ...
 
 input
-  .keyBy(<key selector>)
-  .window(<window assigner>)
-  .process(new MyProcessWindowFunction())
+ .keyBy(_._1)
+ .timeWindow(Time.minutes(5))
+ .process(new MyProcessWindowFunction())
 
 /* ... */