Fix incorrent doc comment for the set method of ContentSize (#9345)

# Objective

This doc comment for the `set` method of `ContentSize`:

```
Set a `Measure` for this function
```
doesn't seem to make sense, `ContentSize` is not a function. 

# Solution

Replace it.

crates/bevy_ui/src/measurement.rs

@@ -55,7 +55,7 @@ pub struct ContentSize {
 }
 
 impl ContentSize {
- /// Set a `Measure` for this function
+ /// Set a `Measure` for the UI node entity with this component
  pub fn set(&mut self, measure: impl Measure) {
  let measure_func = move |size: TaffySize<_>, available: TaffySize<_>| {
  let size = measure.measure(size.width, size.height, available.width, available.height);