MINOR: [R] Add missing PR num to news.md item (apache#43811)

### Rationale for this change

We normally link to somewhere to give the user more context on news items. I noticed the link was missing for this one.

### What changes are included in this PR?

Added PR number to news item.

### Are these changes tested?

No.

### Are there any user-facing changes?

No.

Authored-by: Bryce Mecum <petridish@gmail.com>
Signed-off-by: Jacob Wujciak-Jens <jacob@wujciak.de>

r/NEWS.md

@@ -32,7 +32,7 @@
  functions (UDFs); for UDFs, see `register_scalar_function()`. (#41223)
 * `mutate()` expressions can now include aggregations, such as `x - mean(x)`. (#41350)
 * `summarize()` supports more complex expressions, and correctly handles cases
- where column names are reused in expressions.
+ where column names are reused in expressions. (#41223)
 * The `na_matches` argument to the `dplyr::*_join()` functions is now supported.
  This argument controls whether `NA` values are considered equal when joining. (#41358)
 * R metadata, stored in the Arrow schema to support round-tripping data between