MINOR: [R] Add news bullet for mutate() enhancement in 17.0 (#43189)

### Rationale for this change

I noticed this was missing from the NEWS, and it's kinda important.

### What changes are included in this PR?

A news bullet

### Are these changes tested?

Nope!

### Are there any user-facing changes?

📰

r/NEWS.md

@@ -20,6 +20,7 @@
 # arrow 16.1.0.9000
 
 * R functions that users write that use functions that Arrow supports in dataset queries now can be used in queries too. Previously, only functions that used arithmetic operators worked. For example, `time_hours <- function(mins) mins / 60` worked, but `time_hours_rounded <- function(mins) round(mins / 60)` did not; now both work. These are automatic translations rather than true user-defined 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. 
 * 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 R and Arrow/Parquet, is now serialized and deserialized more strictly. This makes it safer to load data from files from unknown sources into R data.frames. (#41969)