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MINOR: [R][Docs] Fix typo in s3_bucket example code (#40447)

### Rationale for this change

The current code is invalid R code and will error so I'd like to fix that.

### Are these changes tested?

Yes, but manually.

Current code:

```r
>  Sys.setenv("ARROW_S3_LOG_LEVEL","DEBUG")
Error in Sys.setenv("ARROW_S3_LOG_LEVEL", "DEBUG") :
  all arguments must be named
```

New code:

```r
>  Sys.setenv("ARROW_S3_LOG_LEVEL"="DEBUG")
```

### Are there any user-facing changes?

Correct docs.

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>