[SPARK-19828][R][FOLLOWUP] Rename asJsonArray to as.json.array in from_json function in R

## What changes were proposed in this pull request?

This was suggested to be `as.json.array` at the first place in the PR to SPARK-19828 but we could not do this as the lint check emits an error for multiple dots in the variable names.

After SPARK-20278, now we are able to use `multiple.dots.in.names`. `asJsonArray` in `from_json` function is still able to be changed as 2.2 is not released yet.

So, this PR proposes to rename `asJsonArray` to `as.json.array`.

## How was this patch tested?

Jenkins tests, local tests with `./R/run-tests.sh` and manual `./dev/lint-r`. Existing tests should cover this.

Author: hyukjinkwon <gurwls223@gmail.com>

Closes #17653 from HyukjinKwon/SPARK-19828-followup.