[DOCS] Fix error: ambiguous reference to overloaded definition

## What changes were proposed in this pull request?

`df.groupBy.count()` should be `df.groupBy().count()` , otherwise there is an error :

ambiguous reference to overloaded definition, both method groupBy in class Dataset of type (col1: String, cols: String*) and method groupBy in class Dataset of type (cols: org.apache.spark.sql.Column*)

## How was this patch tested?

```scala
val df = spark.readStream.schema(...).json(...)
val dfCounts = df.groupBy().count()
```

Author: Ziyue Huang <[email protected]>

Closes #18272 from ZiyueHuang/master.

(cherry picked from commit e6eb02d)
Signed-off-by: Sean Owen <[email protected]>

docs/structured-streaming-programming-guide.md

@@ -1056,7 +1056,7 @@ Some of them are as follows.
 
 In addition, there are some Dataset methods that will not work on streaming Datasets. They are actions that will immediately run queries and return results, which does not make sense on a streaming Dataset. Rather, those functionalities can be done by explicitly starting a streaming query (see the next section regarding that).
 
-- `count()` - Cannot return a single count from a streaming Dataset. Instead, use `ds.groupBy.count()` which returns a streaming Dataset containing a running count. 
+- `count()` - Cannot return a single count from a streaming Dataset. Instead, use `ds.groupBy().count()` which returns a streaming Dataset containing a running count. 
 
 - `foreach()` - Instead use `ds.writeStream.foreach(...)` (see next section).