Fix possessive "its" in docs and comments (#3998)

This PR fixes a number of places where we were incorrectly using "it's"
where we needed to use the possessive "its".

Release Notes:

- N/A

crates/ui/src/disableable.rs

@@ -1,4 +1,4 @@
-/// A trait for elements that can be disabled. Generally used to implement disabling an element's interactivity and changing it's appearance to reflect that it is disabled.
+/// A trait for elements that can be disabled. Generally used to implement disabling an element's interactivity and changing its appearance to reflect that it is disabled.
 pub trait Disableable {
     /// Sets whether the element is disabled.
     fn disabled(self, disabled: bool) -> Self;