[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 <zyhuang94@gmail.com>

Closes #18272 from ZiyueHuang/master.