[R][Python] Fix cloud storage paths in some documentation

I also see some examples that need updating to the correct bucket in the cookbooks here:
https://arrow.apache.org/cookbook/r/reading-and-writing-data.html#read-a-parquet-file-from-s3

and here:
https://arrow.apache.org/cookbook/py/io.html#reading-partitioned-data-from-s3

Basically this:
`df <- read_parquet(bucket$path("nyc-taxi/year=2019/month=6/data.parquet"))`

Needs to become this:
`df <- read_parquet(bucket$path("nyc-taxi/year=2019/month=6/part-0.parquet"))`

Open the linked R and Python cookbook sections and locate the S3 examples containing `nyc-taxi/year=2019/month=6/data.parquet`. Update those examples to use `nyc-taxi/year=2019/month=6/part-0.parquet`, then verify both linked pages show the corrected bucket path.

Written by the indexing model from the issue text.