### What changes were proposed in this pull request?
This PR is a followup of #27433. It fixes the naming to match with Scala side, and this is similar with https://github.com/apache/spark/pull/31062.
Note that:
- there are a bit of inconsistency already e.g.) `x`, `y` in SparkR and they are documented together for doc deduplication. This part I did not change but the name `zero` vs `initialValue` looks unnecessary.
- such naming matching seems already pretty common in SparkR.
### Why are the changes needed?
To make the usage similar with Scala side, and for consistency.
### Does this PR introduce _any_ user-facing change?
No, this is not released yet.
### How was this patch tested?
GitHub Actions and Jenkins build will test it out.
Also, I manually tested:
```r
> df <- select(createDataFrame(data.frame(id = 1)),expr("CAST(array(1.0, 2.0, -3.0, -4.0) AS array<double>) xs"))
> collect(select(df, array_aggregate("xs", initialValue = lit(0.0), merge = function(x, y) otherwise(when(x > y, x), y))))
aggregate(xs, 0.0, lambdafunction(CASE WHEN (x > y) THEN x ELSE y END, x, y), lambdafunction(id, id))
1 2
```
Closes#31226 from HyukjinKwon/SPARK-30682.
Authored-by: HyukjinKwon <gurwls223@apache.org>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>
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