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MINOR: [R] Fix monospace formatting in dplyr-funcs-doc (#43461)

Added a closing backtick for the `.groups` argument in `summarise()`.

### Rationale for this change

Improves the formatting and appearance of the Acero documentation page at <https://arrow.apache.org/docs/r/reference/acero.html>.

### Are these changes tested?

Yes, I ran `devtools::check()` on this change. I got one warning (related to my environment) and one note that isn't going to be fixable. I would consider this result "passing."

```
❯ checking top-level files ... WARNING
  A complete check needs the 'checkbashisms' script.
  See section ‘Configure and cleanup’ in the ‘Writing R Extensions’
  manual.

❯ checking installed package size ... NOTE
    installed size is 54.7Mb
    sub-directories of 1Mb or more:
      R      5.1Mb
      libs  49.0Mb

0 errors ✔ | 1 warning ✖ | 1 note ✖
```

### Are there any user-facing changes?

This is a documentation change.

Authored-by: Max Feinleib <82004873+feinleib@users.noreply.github.com>
Signed-off-by: Nic Crane <thisisnic@gmail.com>